The Late Triassic Bivalve Monotis in 

Accreted Terranes of Alaska

By N.J. Silberling, J.A. Grant-Mackie, and K.M. Nichols

U.S. GEOLOGICAL SURVEY BULLETIN 2151

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997
For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Silberling, N. J. (Norman John), 1928–

The Late Triassic bivalve Monotis in accreted terranes of Alaska / by N.J. Silberling, J.A. 

Grant-Mackie, and K.M. Nichols.

p. cm.—(U.S. Geological Survey bulletin ; 2151)

Includes bibliographical references.

Supt. of Docs. no. : I 19. 3: 2151

1. Monotis—Alaska. 2. Paleontology—Triassic. 3. Animals, Fossil— 

Alaska. I. Grant-Mackie, J.A. II. Nichols, K. M. (Kathryn Marion), 1946– . 

III. Title. IV. Series.

QE75.B9 no. 2151

[QE812.M6]

557.3 s—dc20

[564’.11] 95-25680

 CIP
III

CONTENTS

Abstract ........................................................................................................................... 1

Introduction..................................................................................................................... 1

Characterization of Alaskan Monotis.............................................................................. 3

Genus Monotis ................................................................................................................ 4

Subgenus Monotis........................................................................................................... 4

Subgenus Pacimonotis .................................................................................................... 5

Subgenus Entomonotis.................................................................................................... 5

Subgenus Eomonotis....................................................................................................... 6

Occurrence of Monotis in Alaska.................................................................................... 8

Northern Alaska ...................................................................................................... 8

East-central Alaska ................................................................................................. 9

Central and Southern Alaska................................................................................... 9

Southwestern Alaska............................................................................................... 10

Southeastern Alaska................................................................................................ 11

Age and Biostratigraphy of Alaskan Monotis Faunas .................................................... 12

Paleogeographic Significance of Alaskan Monotis Faunas ............................................ 14

References Cited ............................................................................................................. 16

Appendix 1. Locality Descriptions ................................................................................ 18

PLATES

[Plates follow appendix]

1. Alexander terrane.

2. Peninsular terrane.

3. Wrangellia terrane.

4. Wrangellia, Susitna, Chulitna terranes.

5. Nixon Fork and McKinley terranes.

6. Non-accretionary Upper Yukon region.

7. Upper Yukon region and Endicott Mountains subterrane.

8. Endicott Mountains subterrane.

9. Endicott Mountains subterrane.

10. North Slope subterrane.

11. North Slope subterrane.

FIGURES

1. Index map showing location of accreted terranes containing Monotis-bearing strata in Alaska and

 adjacent Canada .............................................................................................................................................................. 2

2. Diagrammatic sketch of left valve of Monotis illustrating morphologic features

 and terminology............................................................................................................................................................... 4

3. Chart showing relative stratigraphic ranges of Alaskan Monotis species

 and subspecies ................................................................................................................................................................. 15

4. Checklist showing occurrence of Monotis species in accretionary and non-accretionary Alaska,

 and in the Stikinia terrane of Canada .............................................................................................................................. 16
1

THE LATE TRIASSIC BIVALVE MONOTIS IN 

ACCRETED TERRANES OF ALASKA

By N.J. Silberling, J.A. Grant-Mackie,1 and K.M. Nichols

INTRODUCTION

Thin-shelled, pecten-like bivalves of the genus Monotis

are widespread and locally abundant in Upper Triassic rocks

of Alaska (fig. 1). These easily recognized fossils have been

found in 16 of the 50 accreted tectonostratigraphic terranes

and subterranes recognized by Jones and others (1987) as

forming nearly all of Alaska. Each of these terranes and subterranes is fault-bounded and records a distinct geologic history; their tectonic displacement relative to each other and to

the North American craton ranges from tens to thousands of

kilometers (Jones and others, 1986). Not only are Monotis

faunas useful for dating and correlating the strata of the terranes in which they occur, but the paleolatitudinal distribution of different species of Monotis varied, and thus they also

provide important paleogeographic constraints for interpreting the displacement histories of the various terranes. The

major biogeographic conclusions that can be drawn from

Alaskan Monotis faunas have previously been discussed by

Tozer (1982) and Silberling (1985).

The purposes of this report are to enlarge upon these

general conclusions and to provide needed documentation at

a refined taxonomic level of the Monotis faunas presently

known from Alaskan accreted terranes. Additionally, we

hope that the illustrations, diagnoses, and identification key

will enable Alaskan geologists to use this report as a field

guide to the various species of Monotis. Not only are the

various species useful in the field for general age and biogeographic interpretation but, in places such as northern Alaska,

they also occur in profusion in an easily recognized succession of different species. In such places, they thus can be a

valuable aid in deciphering the structure of intricately

deformed strata.

The Alaskan Monotis faunas on which this study is

based are part of the U.S. Geological Survey paleontological

collections currently housed in the Denver, Colorado, field

center. These collections and their supporting locality and

stratigraphic data represent the effort of many geologists

involved with U.S. Geological Survey research programs in

Alaska since the early 1900’s. In all, Monotis is known from

several hundred geographically or stratigraphically different
2 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

TINTINA FAULT - NORTHERN ROCKY MOUNTAIN TRENCH

DENALI FAULT

70°

65°

170°

60°

160° 150° 140°

130°

55°

60°

120°

65°

130° 120° 70170° 160° 150° 140° °

55°

3

4

5

6

7 8

9

10 11

12

14

13

24

19

23

22

18

16 17

15

25

26

1

2

29

21

28

27 20

AAN

AAN-AAE

AAD

AAE

NX

MK

CH

NN

SU

WR

NY? WR

NY

GD

TG

PE

TK

ST

ST

NAm

AX

AX

AX

CANADA

UNITED STATES

ALASKA
3

Alaskan localities. Specimens figured herein that were collected by the senior author are from localities originally discovered and put into stratigraphic context by E.E. Brabb,

A.F. Buddington, M.C. Churkin, E.M. MacKevett, F.H.

Moffit, R.M. Overbeck, W.W. Patton, Jr., I.L. Tailleur, and

others. Especially significant stratigraphically controlled

Monotis collections from Alaska have been made by M.C.

Churkin, R.L. Detterman, C.G. Mull, I.L. Tailleur, and H.A.

Tourtelot.

The selection and preparation of materials for this study

and the interpretation of their geologic settings were largely

the work of Silberling and Nichols, whereas the formulation

of taxonomic concepts and final identifications of the specimens illustrated herein were for the most part done by GrantMackie during a visit to the USGS Denver field center of

about 2 months in late 1985 as part of a Fulbright Travel

Grant.

CHARACTERIZATION OF

ALASKAN MONOTIS

Fourteen different kinds of Monotis are presently

known from Alaska. All of these are characterized and illustrated in the present study, even though some are regarded

only as subspecies and some remain indefinite in terms of

specific or subspecific assignment. Representation of the

various taxa varies widely; some are extremely common,

their shells occurring locally in rock-forming abundances,

whereas others are known only from one or two specimens.

Because one of the principal objectives of this study is to

record the Monotis faunas of the various Alaskan terranes,

specimens that document the occurrence of a taxon in a certain terrane are illustrated even though they may not be particularly well preserved. Nevertheless, for each species or

subgenus the most morphologically complete specimens

available in the Alaskan collections are figured and characterized. Some taxa are illustrated from more than one terrane

or subterrane.

Taxonomic assignments adopted here are strictly circumscribed by means of measureable parameters and the

presence or absence of particular morphologic features. The

terminology used to describe the morphology of Monotis

shells is illustrated in figure 2. Although historically one of

us (NJS) has had a proclivity towards population-based,

morphologically variable species concepts, the more operational biometric approach advocated by Grant-Mackie

(Grant-Mackie and Silberling, 1990) is followed here. This

approach offers the two-fold advantage of greater objectivity

and of making the various taxa considered here directly comparable to those characterized by Grant-Mackie from elsewhere on the Pacific margin, principally New Zealand

(Grant-Mackie, 1981). Taxonomic descriptions of the subgenera, and of some of the species and subspecies, of

Alaskan Monotis have been published recently by GrantMackie and Silberling (1990), and the reader is directed to

this paper for systematic and biometric data.

The genus Monotis is morphologically distinct from

other kinds of generally pecten-like bivalves and is generally

placed by itself in the separate Family Monotidae. Recognition of four subgenera within the genus is useful in that the

subgenera themselves have temporal and paleogeographic

significance. Starting with the genus Monotis itself, the various subgenera, species, and subspecies of Alaskan Monotis

are briefly diagnosed and discussed herein, and a key to their

identification is provided following.

1. Adult size small to medium; posterior ear ribbed.

Subgenus M. (Eomonotis) ................................................... 2

Medium to large size; posterior ear smooth ....................... 9

2. Left valve much more inflated than right................................. 3

Left valve only little more inflated than right .......................... 4

3. Ribs very fine and dense; about 40 primary ribs

................................................ n.sp. aff. M. (Eo.) inaequivalvis

Ribs stronger; about 18 primary ribs.....................M (Eo.) jakutica

4. Ribs fine, narrow, relatively sharp ............................................ 5

Ribs broader, rounded ............................................................... 6

5. Ribs crowded; about 25 primary ribs; secondary ribs of

similar strength................................................... M. (Eo.) typica

Ribs well differentiated in three orders; about 30 primary

ribs................................................................. M. (Eo.) pinensis

6. Secondary ribs inserted early.................................................... 7

Secondary ribs inserted late ...................................................... 8

7. Primary ribs about 30; secondary ribs inserted at mid-height;

anterodorsal margin “pouts” ..............M. (Eo.) daonellaeformis

Primary ribs about 20; faint tertiary ribs inserted

marginally................................. M. n.sp. aff. M. (Eo.) anjuensis

8. Primary ribs 25–30; ribs tend to fade

dorsally ...................................................M. (Eo.) obtusicostata

Primary ribs about 30; ribs do not weaken

dorsally ........................................................ M. (Eo.) anjuensis

9. Ribs narrow and sharp; subequivalved, left valve little more

inflated than right ................................................................ 10

Ribs relatively coarse; left valve much more inflated than

right .............Subgenus Entomonotis................................... 13

10. Valve obliquely oval; primary ribs 18 or more

Subgenus Monotis............................................................... 12

Valve subcircular to posteroventrally elongate; different

orders of ribbing well differentiated

Subgenus Pacimonotis......................................................... 11

CHARACTERIZATION OF ALASKAN MONOTIS

Figure 1 (facing page). Index map showing location of accreted

lithotectonic terranes containing Monotis-bearing strata in Alaska

and adjacent parts of Canada. Numbered solid dots, localities of

specimens illustrated on plates 1–11; numbered ×’s, other significant occurrences of Monotis discussed in text. NAm (shaded area),

non-accretionary Phanerozoic North American rocks. Symbols for

accreted terranes: AX, Alexander terrane; AAD, DeLong Mountains

subterrane, Arctic Alaska terrane; AAE, Endicott Mountains subterrane, Arctic Alaska terrane; AAN, North Slope subterrane, Arctic

Alaska terrane; AANAAE, subterrane transitional from AAN to

AAE; CH, Chulitna terrane; GD, Goodnews terrane; MK, McKinley

terrane; NN, Nenana terrane; NX, Nixon Fork terrane; NY, Nyac

terrane; PE, Peninsular terrane; ST, Stikinia terrane; SU, Susitna

terrane; TG, Togiak terrane; TK, Tikchik terrane; WR, Wrangellia

terrane.
4 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

DORSAL

PREUMBONAL

 LENGTH

POSTUMBONAL LENGTH

LENGTH

HEIGHT

VENTRAL

POSTERIOR

ANTERIOR

posterior truncation

 angle

posterior sinus

posterior ear

length of 

 post. ear

umbo (or beak)

primary rib

second-order rib

third-order rib

hingeline

11. About 15 primary ribs; concentric striae conspicuous, especially

in interstices between ribs.........................M. (P.) subcircularis

12. Primary ribs about 20; concentric striae commonly

present................................................................. M. (M.) haueri

Primary ribs about 30; concentric striae rarely

present.............................................................M. (M.) alaskana

13. Ribs moderately narrow, well differentiated........................ 14

Ribs broad, rounded; about 16 primary ribs, few secondary

ribs............................................................M. (En.) pachypleura

14. About 14 primary ribs........................... M. (En.) ochotica ochotica

About 18 primary ribs.................M. (En.) cf. ochotica densistriata

Genus MONOTIS Bronn, 1830

Diagnosis.—Pecten-like bivalves that are thin shelled

relative to their length (fig. 2) which rarely exceeds 100 mm;

mature shells of the smallest species are about 20 mm in

length. Viewed end on, subequivalved to strongly inequivalved, wherein the left valve may be much more inflated

than the right, which may be nearly flat. In lateral view, outline of the shell subquadrate, subcircular, or obliquely suboval; usually inequilateral in shape having the beak shifted

anteriorly and the greatest height shifted posteriorly. Dorsal

margin straight and comarginal with a distinct posterior ear

that is separated from the posterodorsal margin by the concave-outward curve of the posterior sinus (see fig. 2). Depth

of posterior sinus varies from shallow to deep, and the posterior angle (fig. 2) varies from 90° to 160°, depending on the

subgenus. Anterior (or byssal) ear on right valve relatively

very small and projected inwards so as to be inconspicuous;

no anterior ear on left valve on which the anterodorsal margin is evenly rounded. Sculpture of rounded, regular, radial

ribs that in a few species may become obsolete with growth

or be absent. On ventrolateral surface two or more orders

(that is, primary, secondary, tertiary, and so on) of radial ribs,

generally of distinctly different strength, produce an alternating pattern of coarse-fine-coarse-fine, and so on, or coarsefine-medium-fine-coarse-fine, and so on. In some species

ribs and interspaces are crenulated by fine, evenly spaced,

concentric growth striae (for example, pl. 2, fig. 13). Posterior ear may bear radial ribs or be smooth, depending on the

subgenus.

Remarks.—Specimens of Monotis, even where preserved only as flattened impressions, are nearly

unmistakable among other kinds of bivalve fossils by virtue

of their size, characteristic shape that results from the prominent posterior ear, and radial ribbing. Although not strictly

true, the inconspicuous nature of the anterior ear, as compared to the posterior ear, gives the genus its name, meaning

“one-eared.” In the older literature, the common North

American species Monotis subcircularis was generally

included in the typically late Paleozoic genus Pseudomonotis, meaning “falsely one-eared,” because the inconspicuous

anterior ear was noticed on this New World species long

before it was discovered on European species originally

included in the exclusively Late Triassic genus Monotis.

Subgenus MONOTIS Bronn, 1830

Diagnosis.—Small- to medium-size Monotis having an

obliquely oval outline. Subequivalved, the left valve being

only a little more inflated than the nearly flat right valve.

The posterior ear is smooth and well separated from the rest

of the valve by a moderately deep posterior sinus; posterior

angle 130°+15°. Radial ribs are fine and characteristically

tend to weaken and become wavy posterodorsally. Concentric striae not always present.

Remarks.—This subgenus is distinguished by its modest size, notably oblique shape, relatively abundant, regular

ribs, and smooth, well-differentiated posterior ear.

MONOTIS (MONOTIS) ALASKANA Smith, 1927

2Plate 2, figures 1–7; plate 3, figures 4–7, ?14–15; plate 4, figures 6–7

Diagnosis.—Medium size M. (Monotis) up to 50 mm

long. Postumbonal length up to twice anterior length and

2–3 times length of posterior ear that is truncated posteriorly

at 130°+15°. Primary ribs number 30+5; secondary ribs

inserted at valve height of about 10 mm, and tertiary ribs, if

present, irregularly inserted at valve height of about 25 mm.

Concentric sculpture closely spaced and faint, if visible at

all.

Remarks.—Within the subgenus this species is distinguished by its numerous, narrow, crowded ribs and weak

development of concentric striae. At times specimens of M.

2A query (?) before a plate figure number indicates some uncertainity

in the identification of the particular specimen(s). It is used in lieu of the

designation “cf.” where Monotis specimens of both certain and uncertain

species assignment are listed under the same species-name heading in the

text.

Figure 2. Diagrammatic left valve of a relatively inflated

Monotis shell illustrating principal morphologic features and

morphologic terms used herein.
5

(M.) alaskana from Alaska and elsewhere in North America

have been referred to the typically Tethyan species M. (M.)

salinaria (Schlotheim), the type species of the genus and

subgenus, and M. salinaria has been regarded as a senior

synonym of M. alaskana. However, careful comparison of

Monotis in the extensive collections of the U.S. Geological

Survey with topotype specimens of M. salinaria from Siriuskogel, Austria (pl. 4, figs. 11–13), shows that none of the

North American specimens belongs in this species, which

has about 22 primary ribs compared to about 30 in M.

alaskana (Grant-Mackie and Silberling, 1990).

MONOTIS (MONOTIS) HAUERI Kittl, 1912

Plate 3, ?figures 8–13; plate 8, figure 9

Diagnosis.—Moderately large Monotis s.s. up to 70 mm

in length. Outline markedly oblique with the postumbonal

length as much as four times the preumbonal length. Primary

ribs number 17–20 and are strongly developed compared to

the secondary ribs. Regular, dense, concentric striae

commonly preserved.

Remarks.—This species differs from M. alaskana in

having stronger and many fewer primary ribs, more conspicuous concentric ornamentation, and on the average a more

obliquely oval shape. This species and M. subcircularis are

easily confused, especially in poorly preserved or incomplete

material, because of similarities of inflation, outline, and

sculptural style. They are readily separated, however, by the

small difference in the average number of primary ribs, intermediate population samples not being represented. Some

previously published identifications of North American

specimens as M. subcircularis should be reassigned to M.

haueri as defined biometrically by Grant-Mackie and Silberling (1990), but this change would not affect geologic

interpretations based on these specimens.

Subgenus PACIMONOTIS Grant-Mackie and Silberling, 1990

Diagnosis.—Moderate- to large-size Monotis having a

subcircular to posteroventrally elongate outline. Subequivalved, both valves being only gently inflated. Posterior ear

conspicuous, unsculptured, well separated from posterodorsal margin by a broad, deep sinus having a relatively acute

posterior angle. Strong, narrow, abruptly rounded primary

radial ribs differentiated from distinctly weaker subsidiary

orders of ribbing. Most members have conspicuous dense,

regular, concentric striae, especially in interstices between

primary ribs.

Remarks.—The size, the distinct, smooth posterior ear,

the subequivalve shape, and the nature of the ornamentation

distinguish this subgenus. In Alaska it is represented only by

M. (Pacimonotis) subcircularis; other species of the subgenus occur elsewhere around the circum-Pacific margin

(Grant-Mackie and Silberling, 1990).

MONOTIS (PACIMONOTIS) SUBCIRCULARIS Gabb, 1864

Plate 1, figures 4–10, 13–14, 17–21; plate 2, figures 8–13; plate 3, figures 

1–3, ?18–19, 20; plate 4, figures 1–5, ?8–10; plate 5, figures 1–4; plate 7, 

figures ?1–3, 4–5; plate 8, ?figures 1–2; plate 9, figures 5–6; plate 11, ?figure 13

Diagnosis.—Large (up to 120 mm long) valves of subcircular to suboval outline. Postumbonal length about 1.5

times the anterior length and 2–4 times length of the posterior ear. Posterior angle 120°+10°. Number of primary ribs

15+5; secondary ribs inserted early, at valve heights of 5–15

mm, tertiary ribs at valve heights of 20–30 mm; fourth-order

ribs irregularly present on valves larger than 45 mm high.

Concentric striae conspicuous.

Remarks.—This is the largest, most widespread, and

commonest North American Monotis, characterized by its

regular concentric sculpture, low primary rib count coupled

with a high total number of ribs, and relatively small, but distinct posterior ear having a low posterior truncation angle.

Subgenus ENTOMONOTIS Marwick, 1935

Diagnosis.—Medium- to large-size Monotis of

obliquely suboval outline. Strongly inequivalve, having a

conspicuously inflated left valve, the umbo of which projects

dorsally well above the hingeline, and a flat to slightly

inflated right valve possessing a much less conspicuous

umbo. Posterior ear moderate to proportionally large,

smooth, well separated from the rest of the valve by a distinct

posterior sinus. Radial ribs relatively broad, rounded, and

well differentiated; on some species ribs weaken and fade

posteriorly. Concentric striae generally absent or poorly

developed.

Remarks.—Distinguishing features of this subgenus are

its strongly inequivalved shape and coarse ribbing.

MONOTIS (ENTOMONOTIS) PACHYPLEURA (Teller, 1886)

Plate 9, figures 1–4, 9–11

Diagnosis.—Medium size (up to 50 mm long),

obliquely oval; postumbonal length 1.3–2.3 times the

preumbonal length and about 2.5–3.0 times length of posterior ear. Inequivalve; inflation of right valve about half that

of left. Ribs number 20+7, with an average of 16–17 broad,

low, rounded primary ribs and a few much narrower, weaker,

secondary ribs at the posteroventral margin. Interstices

between ribs equal to or slightly narrower than primaries.

Remarks.—From other Alaskan species of this subgenus, M. (En.) pachypleura differs in its coarse, rounded ribbing, dominated by primary ribs.

MONOTIS (ENTOMONOTIS) OCHOTICA OCHOTICA

(Keyserling, 1848)

Plate 9, figures 8, 12–13; plate 11, figures 1–3, ?4–7, 14–15

Diagnosis.—Medium- to large-size M. (Entomonotis)

up to 70 mm long, obliquely oval, left valve well inflated,

right valve almost flat. Beak at about anterior third of valve

length; posterior valve length 2–3 times length of posterior

CHARACTERIZATION OF ALASKAN MONOTIS
6 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

ear. Ribs well spaced, rounded but relatively narrow; 14+2

primary ribs; secondary ribs inserted early, weak tertiary ribs

inserted later at 20–35 mm valve height. Fine concentric

striae rarely present.

Remarks.—Alaskan specimens show the essential features of the typical Siberian M. (En.) ochotica sensu stricto

and are assigned to this subspecies with confidence. They

possess fewer and coarser ribs than M. (En.) o. densistriata,

however defined, as discussed below, and yet have much

narrower and more densely spaced ribs than M. (En.)

pachypleura.

MONOTIS (ENTOMONOTIS) sp. cf. M. (E.) OCHOTICA 

DENSISTRIATA (Teller, 1886)

Plate 1, figures 1–3, 11–12

Diagnosis.—Small- to medium-size Monotis up to 40

mm long. Subcircular to obliquely suboval in outline,

strongly inequivalve. Beak at the anterior third of the valve

length; posterior valve length 2–4 times length of posterior

ear. Ribs relatively sharp and narrow for M. (Entomonotis);

primary ribs number 18+3, much weaker secondary ribs

inserted at valve height of 10+5 mm, and some faint tertiary

ribs inserted ventrally.

Remarks.—M. (En.) o. densistriata has been widely

reported in the northern and western circum-Pacific, but

examination by Grant-Mackie (unpub. data) of specimens so

identified strongly suggests that many of these reported

occurrences are incorrect. Misconceptions concerning this

subspecies make its identification difficult outside of the

former U.S.S.R., from where it was originally described.

The form diagnosed above is smaller than many Asian specimens assigned to M. o. densistriata (for example, those

illustrated by Kiparisova and others, 1966) and seems to be

relatively longer and have a significantly larger posterior

truncation angle of the posterior ear. Characteristics of the

ribbing and inflation, however, are similar. The Alaskan

form also has a higher posterior truncation angle of the posterior ear and a more inflated left valve than Canadian specimens assigned to the subspecies by Westermann (1962).

Grant-Mackie, thus, would regard the Alaskan specimens as

similar, rather than identical, to M. (En.) o. densistriata,

pending better characterization of the Asian type material.

Silberling, on the other hand, would treat the specimens figured here simply as immature variants of M. (Pacimonotis)

subcircularis with which they occur in the same bed and

with whose immature shells they might intergrade in the

degree of inflation of the left valve and nature of the ribbing.

This interpretation assumes that strongly inflated immature

variants of M. subcircularis would be precluded from maturing and thus would be represented only by small shells.

Subgenus EOMONOTIS Grant-Mackie, 1978

Diagnosis.—Monotis of small or medium size, almost

equivalve to moderately inequivalve, and with fine radial

ribs that extend onto the posterior ear. Posterior ear is poorly

separated from the rest of the shell, the posterior sinus being

shallow or absent. Concentric sculpture is absent.

Remarks.—The prime morphologic distinctions of this

subgenus are its poorly differentiated, ribbed posterior ear

and the fine, numerous radial ribs.

MONOTIS (EOMONOTIS) ANJUENSIS Bytschkov and Efimova, 1966

Plate 5, figures 11–16

Diagnosis.—Medium-size Monotis up to 40 mm long.

Tranversely to obliquely oval, subequivalve, and of low

inflation, with both umbones projecting slightly dorsally of

the hingeline. Primary ribs strong, rather broad, rounded,

closely set, and number 30–36; secondary ribs much weaker

and posteroventrally inserted late in some interspaces. Postumbonal length about 1.5–2.5 times preumbonal length.

Posterior ear about one-half posterior length, ribbing on ear

not always visible.

Remarks.—Alaskan specimens of this species were

first recorded from North America by Grant-Mackie and Silberling (1990) who pointed out that they cannot be separated

from the typical Asian examples by any significant feature.

They were placed in the subgenus Eomonotis because the

unexcavated, poorly separated posterior ear clearly bears

radial ribs at least on some specimens, although the ears of

some internal casts appear smooth. M. (Eo.) anjuensis differs from M. (Eo.) typica and pinensis in its coarser ribbing

and more nearly transverse outline, which is notably longer

than high. From M. (Eo.) inaequivalvis and similar forms, it

differs both in these same features and in its less inflated left

valve; it differs from M. (Eo.) daonellaeformis by its higher

primary rib count and by the shape of the anterior margin that

does not project above the hingeline. It is closest to M. (Eo.)

obtusicostata but has better formed, more numerous primary

ribs and a more transverse outline.

MONOTIS (EOMONOTIS) n. sp. aff. M. (EO.) ANJUENSIS 

Bytschkov and Efimova, 1966

Plate 11, figures 17–18

Diagnosis.—Medium-size M. (Eomonotis) up to 50

mm long, obliquely oval, subequivalved, and of low inflation. About 20 broad, rounded, primary ribs; weaker secondary ribs inserted early in most interspaces, and faint tertiary

ribs near the margin in the posteroventral quadrant.

Remarks.—This form, known from only one locality,

shows many similarities to M. (Eo.) typica in outline, size,

and ribbing, but the primary ribs are much more prominent

and the ribbing better differentiated. It is much closer to M.

(Eo.) anjuensis, especially in rib form, but it has fewer primary ribs, and the secondary ribs appear earlier. From M.

(Eo.) obtusicostata it is separable by these same characters.

MONOTIS (EOMONOTIS) DAONELLAEFORMIS Kiparisova, 1960

Plate 6, figures 1–5

Diagnosis.—Medium size (up to 40 mm long). Shape

transversely oval, with anterodorsal margin projecting
7

dorsally above the projection of the hingeline. Beaks inconspicuous and valves of subequal, low inflation. Postumbonal

length 1.5–2.0 times preumbonal length. Sculpture of about

30 prominent, rounded, primary radial ribs; secondary ribs

much weaker, inserted at about half valve height. Anterior

ribs, and sometimes also posterior ribs, curve dorsally.

Remarks.—This species is characterized by its transversely oval outline, dorsally projected anterodorsal margin,

and moderately broad, rounded, primary ribs that, however,

are narrower than those of M. obtusicostata. Westermann

(1966) identified M. cf. daonellaeformis from the Canadian

Arctic, but the specimens to which he refers are not readily

related to this species because of their sparse ribbing and

greater left-valve inflation.

MONOTIS (EOMONOTIS) n. sp. aff. M. (EO.) 

INAEQUIVALVIS Bronn, 1930

Plate 10, figure 5

Remarks.—This unique form is represented by a single

small (25 mm long), obliquely oval, well-inflated (8 mm in

width) left valve having closely spaced, fine ribs of equal

strength, differentiated only by their level of origin. Forty

primary ribs commence near the beak, and eight secondary

ribs are then inserted in the ventral quadrant at mid height.

The posterior ear is finely ribbed, set off by a distinct posterior sinus, and about 1/3 the posterior length of the shell,

which is about 21

/2 the anterior length.

This shell is especially characterized by its high primary rib count and inflation. It is most closely similar to the

European M. inaequivalvis and M. hoernesi Kittl, differing

from the former by its greater number of primary and fewer

secondary ribs and from the latter by its oval outline, greater

inflation, and fewer primary ribs. Westermann (1962)

described “Monotis n. sp.? aff. inaequivalvis” from northeastern British Columbia, but his form is distinct from ours

in its still greater number of primaries (about 50) and in its

more prominent posterior ear.

MONOTIS (EOMONOTIS) JAKUTICA (Teller, 1896)

Plate 3, figures 16–17; plate 5, figures 7–10;

plate 10, figures ?1–4, ?6, ?10–11

Diagnosis.—Small to medium size (up to 30 mm long),

subcircular to obliquely oval, with left valve strongly

inflated and right valve almost flat. Beak situated at about

the anterior third of the valve length. Sculpture of 18 + 4

strong, high, sharp, well-spaced primary ribs, much weaker

secondary ribs inserted early, and in some specimens very

weak tertiary ribs inserted marginally in a few interspaces.

Posterior ear bears 2–5 fine ribs and is 1/2–1/3 of posterior

valve length.

Remarks.—This species is commonly associated with

M. subcircularis in North American collections but has seldom been recorded. The marked inflation of the left valve

and the coarser and larger number of primary ribs readily

separate M. jakutica from juvenile M. subcircularis. These

same characters separate M. (En.) ochotica densistriata from

M. subcircularis, but the former differs from M. jakutica in

having still more delicate ribbing (compare pl. 1, figs. 1–3,

with pl. 5, fig. 7) and a smooth posterior ear. The strongly

inequivalved shape also clearly differentiates M. jakutica

from other North American Eomonotis except for M. n. sp.

aff. M. (Eo.) inaequivalvis that has twice as many and much

finer primary ribs.

MONOTIS (EOMONOTIS) OBTUSICOSTATA Westermann, 1966

Plate 10, figures 7–9, 12–14; plate 11, figures 10–12

Diagnosis.—Medium-size (up to 40 mm long),

obliquely oval, subequivalve, of low inflation, with umbo of

the left valve projecting slightly above hingeline. Sculpture

of 25–30 broad, rounded, well-spaced, primary ribs, with

weak secondary ribs inserted near the ventral margin. Ribs

tending to weaken anterodorsally and posterodorsally. Posterior shell length about three times the anterior shell length,

which approximately equals the length of the posterior ear.

Remarks.—This species is readily separated from most

species of M. (Eomonotis) by its coarser, well differentiated

ribbing. It is similar in shape to M. (Eo.) anjuensis but differs in having, on the average, fewer ribs and in exhibiting

distinct fading of the ribs towards both the anterior and posterior dorsal margins.

MONOTIS (EOMONOTIS) PINENSIS Westermann, 1962

Plate 1, ?figure 16; plate 5, ?figures 18–22; plate 6,

 ?figures 6–10, 12–16; plate 11, figures ?8–9, 16

Diagnosis.—Medium size (up to 40 mm long),

obliquely oval shells of low inflation. Left-valve umbo

prominent, projecting significantly above hingeline; rightvalve umbo much less prominent, barely projecting beyond

hinge. Ribs fine, well differentiated, quite well spaced; primary ribs number 30 + 6; secondary ribs inserted in most

interspaces, some tertiary ribs inserted marginally. Posterior

ear 1/2 to 1/3 posterior length of valve, which is 1.5–2.5

times the anterior length.

Remarks.—Few Alaskan specimens can be positively

assigned to this species, although imperfectly preserved similar and probably conspecific forms are locally common.

The diagnosis given above is taken from Westermann’s original description and from topotype collections. Monotis

(Eo.) pinensis is most closely similar to M. (Eo.) typica but

has a slightly more inflated left valve, more primary ribs,

better differentiated primary and secondary ribs, and a better

separated posterior ear. From other forms of M. (Eomonotis)

it is distinguished by its low inflation and (or) fine ribbing.

MONOTIS (EOMONOTIS) TYPICA (Kiparisova, 1936)

Plate 5, ?figure 5; plate 8, figures ?3–4, 5–8, ?10,

13–17; plate 9, ?figure 7; plate 11, figures 19–20

Diagnosis.—Medium size (up to 40 mm long),

obliquely oval, with up to 70 fine, closely spaced ribs of two

CHARACTERIZATION OF ALASKAN MONOTIS
8 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

orders. Primary ribs number 22–29; secondary ribs of subequal strength. Posterior shell length 2–3 times anterior shell

length. Right valve very little inflated, beak not projecting

above the hingeline; left valve a little more inflated, the beak

projecting slightly dorsal to the hingeline.

Remarks.—This species, originally named by Kiparisova (1936) as a variety of “Pseudomonotis scutiformis” from

the basal beds of the Monotis sequence in the Russian Far

East, is well represented in Alaska. It is distinguished

primarily by the weakly differentiated posterior ear, the posterior sinus being shallow, if perceptible at all, and by the

numerous ribs among which the primary and secondary ribs

are poorly differentiated.

OCCURRENCE OF MONOTIS IN 

ALASKA

The accreted tectonostratigraphic terranes of Alaska

and adjacent parts of Canada from which Monotis is known

are shown in figure 1 (after Jones and others, 1987; Monger

and Berg, 1987). Monotis also occurs in the relatively small

area of east-central Alaska, which is interpreted as part of the

North American Continent that was not accreted during

Phanerozoic time. Numbered localities in figure 1 are either

those from which the specimens figured herein were collected or those discussed in the text where other tectonically

or paleogeographically significant occurrences of Monotis

are known.

NORTHERN ALASKA

In northern Alaska several different species of Monotis

are abundantly represented in Upper Triassic rocks of the

North Slope, Endicott Mountains, and DeLong Mountains

subterranes of the Arctic Alaska terrane. According to Bodnar (1984), the Monotis-bearing strata are condensed, coldwater, anoxic to hypoxic deposits representing an original

northeast-southwest (in present coordinates) progression to

deeper, below wave-base, depositional environments. These

rocks have been structurally imbricated, with marked stratigraphic juxtaposition at subterrane boundaries, during Early

Cretaceous north-verging thrust faulting.

In outcrops of the North Slope subterrane in northeastern Alaska, as at localities 3 and 4 (fig. 1), Monotis occurs in

black, sooty shale and concretionary limestone at the top of

the Shublik Formation and in the basal meter, or so, of the

overlying Karen Creek Sandstone as described by Detterman

and others (1975)3

. Monotis is also well represented in well

3In the list of fossil collections given by Detterman and others (1975,

p. 43) the description of collection number 1 refers to collection number 3

as plotted in their figure 6, and number 3 in their list refers to number 1 in

their figure. 

cores from equivalent strata in the subsurface of the North

Slope, both from the Prudhoe Bay area (on the coastline

northwest of loc. 3, fig. 1) and the National Petroleum

Reserve (for example, locs. 1 and 2, fig. 1).

In the Endicott Mountains subterrane of the Arctic

Alaska terrane (locs. 8, 10–11, fig. 1), Monotis is locally

abundant in the limestone member and upper part of the

chert member of the Otuk Formation, as described, for

example, by Mull and others (1982),4

 Bodnar (1984), and

Blome and others (1988). These two members of the Otuk

are the approximate age equivalents of the Shublik

Formation in the North Slope subterrane. They are mainly

oxygen-deficient, below wave-base marine deposits in

which Monotis is associated with few others kinds of fossils

besides halobiid bivalves, radiolarians, and conodonts. The

depositional setting of the most deep-water parts of the Otuk

was interpreted by Bodnar (1984) as “an outer shelf, a very

low-angle slope, or *** an epicontinental sea on a broad continental basement in up to inner bathyal water depths.” In the

Point Hope–Lisburne Hills region at the western end of

northern Alaska (locs. 5, 6, fig. 1), the rocks containing

Monotis are also assigned to the Otuk Formation, but the

underlying Paleozoic section has stratigraphic characteristics transitional between the Endicott Mountains and North

Slope subterranes. Farther east in the structurally complex

Brooks Range, some Otuk Formation localities, such as

locality 9 (fig. 1), may be within the still more offshore upper

Paleozoic and Triassic rocks of the structurally higher

DeLong Mountains subterrane.

Monotis faunas in the various subterranes of the Arctic

Alaska terrane include most of the same species and subspecies and have about the same diversity. Specimens either

definitely or provisionally assignable to Monotis (Entomonotis) ochotica ochotica, M. (Pacimonotis) subcircularis,

M. (Eomonotis) pinensis, M. (Eo.) obtusicostata, and M.

(Eo.) typica locally occur in profusion. However, despite

their abundance, well-preserved specimens and unbroken

biostratigraphic sequences are relatively few, due to the

prevalent intense, fold-and-thrust style of deformation and

the general lack of good exposure. Monotis (Entomonotis)

pachypleura is well represented only in the westernmost

Otuk Formation exposures. The Monotis-bearing sequence

in these exposures may preserve younger beds whose age

equivalents are cut out disconformably at the top of the limestone member farther east in the Otuk Formation and at or

just above the top of the Shublik Formation of northeast

Alaska where either M. (P.) subcircularis (or the closely

similar M. (M.) haueri) or M. (Entomonotis) ochotica ochotica are the youngest Monotis and occur, at most, a few

meters above middle Norian species of Eomonotis.

4Note that the subgeneric names Eomonotis and Entomonotis are

applied partly to the wrong species on p. 360 of Mull and others (1982).
9

The occurrence of another common fossil, the Ladinian

flat clam Daonella frami, in the basal part of the Shublik and

near the boundary between the limestone and shale members

of the Otuk, poses a pitfall for geologists studying these condensed, highly deformed, imperfectly exposed strata that

commonly crop out as partial overturned sections. The pattern and strength of ribbing of D. frami are remarkably similar in negative impressions to that in positive impressions of

species of Monotis, such as M. subcircularis, that bear ribs

of distinctly different orders, and vice versa. The two different kinds of bivalves have on occasion been confused on the

basis of fragmentary, poorly preserved material. As a consequence, the younger and older parts of Triassic sections

sometimes have been mistaken for one another (Patton and

Tailleur, 1964).

EAST-CENTRAL ALASKA

Monotis-bearing rocks on and near the Yukon River in

Alaska (locs. 13, 14, fig. 1), near the Canadian border, are

part of a condensed Middle and Upper Triassic sequence of

dark, bituminous shale and impure limestone that may be

less than 100 m in total thickness and forms the base of the

Glenn Shale (Brabb, 1969). Because of the recessive nature

and local structural complexity of these rocks, complete sections through even the relatively thin Monotis-rich part of the

sequence are not exposed. Isolated, lithologically and faunally similar strata crop out in a few places in the Mackenzie

fold belt 100–400 km to the east in Canada (Mountjoy,

1967). These rocks, in turn, are similar to the Late Triassic

continental-slope deposits that are well represented in the

Northern Rocky Mountains thrust belt of northeastern British Columbia. Other parts of the pre-Tertiary section also

show transitions from the intricately thrust-faulted exposures

near the Alaska-Canada border eastward into the North

American Shelf. Hence, the east-central Alaskan occurrences of Monotis are regarded as belonging to a tectonically

disrupted continuation of the continental margin and not to

an accretionary terrane.

In the east-central Alaskan strata, Monotis shells are

commonly so abundantly stacked together that compression

has impressed one shell into another forming a confused pattern of interfering ribbing and incomplete shell outlines (pl.

6, fig. 13; pl. 7, figs 1–2). The abundance of shells themselves makes it difficult to obtain positively identifiable

specimens. Species of Eomonotis, such as M. (Eo.) daonellaeformis (listed by Silberling as M. “scutiformis typica” in

Brabb, 1969, p. 112) and probably M. (Eo.) pinensis, are particularly well represented. In younger beds, specimens that

probably belong to M. (P.) subcircularis are well represented, although invariably poorly preserved. Other late

Norian species are not known, most likely because of lack of

exposure.

CENTRAL AND SOUTHERN ALASKA

Monotis has been collected from several different terranes in the Kuskokwim Mountains, Alaska Range, and

Wrangell Mountains of central and southern Alaska. These

terranes lie both north and south of the Denali fault (fig. 1),

a major strike-slip fault that in this part of Alaska follows a

Mesozoic suture zone (Nokleberg and others, 1985).

Occurrences of Monotis-bearing rocks north of the

Denali fault in central Alaska are very limited. In the areally

extensive Nixon Fork terrane, Triassic strata, along with Permian rocks, have been found only in one small area (loc. 15,

fig. 1). Pre-Cretaceous rocks forming the major share of the

Nixon Fork terrane are Ordovician through Devonian shelf

carbonate rocks inferred to overlie Precambrian metasedimentary rocks. The total thickness of dated Triassic rocks,

all of which are apparently Upper Triassic, is estimated by

Patton and others (1977) to be about 60 m. Gradationally

above these are another 100 m of bedded chert that is latest

Late Triassic and possibly Jurassic in age. In the Upper Triassic sequence beneath the chert, Monotis occurs at several

different levels within 5–10 m of incompletely exposed,

platy weathering, silty or finely sandy, phosphatic, spicular

limestone (pl. 5). Thicker and more massive interbeds

within and just above the fossiliferous part of the sequence

are impure bioclastic packstones and grainstones, formed

largely of brachiopod, mollusk, and echinoderm fragments,

and intraclastic sedimentary breccias containing abundant

dark clasts of partly dolomitized phosphatic rock. These

massive beds are interpreted as debris-flow deposits within a

deep-water sequence The only megafossils preserved are

pelagic forms, such as Halobia, Monotis, and the enigmatic,

spherical, planktonic(?) hydrozoan Heterastridium. The fossiliferous part of the section grades upward though a few tens

of meters of notably spiculitic impure limestone into the bedded chert unit.

At one of the two fossiliferous exposures, M. (Eomonotis) jakutica and at least one other kind of Eomonotis occur

in float from USGS Mesozoic loc. D11286 about 4–5 m

below M. (Pacimonotis) subcircularis from USGS Mesozoic

locality D11285; at the other exposure (USGS Mesozoic loc.

D11286), about 3 km away, M. (Eomonotis) anjuensis

occurs in bedrock several meters below M. (P.) subcircularis

and Heterastridium. The occurrence here of the spherical

hydrozoan Heterastridium is noteworthy because of its possible biogeographic significance; this is its northernmost

known occurrence in North America.

Southeast of the Nixon Fork terrane, and only about 2

km north of the Denali fault at locality 16 (fig. 1), Monotis

has been found in one place within the McKinley terrane (pl.

5, figs. 17–22). Here, Monotis provisionally assigned to M.

(Eomonotis) pinensis is abundant along with Halobia cf. H.

fallax in a slab of black, laminated limestone, about 0.1 by

1.0 m in cross section, within Upper Jurassic(?) to Cretaceous flysch. This slab was found at an Early Cretaceous

OCCURRENCE OF MONOTIS IN ALASKA
10 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

Inoceramus and belemnoid locality (map locality no. 4 of

Reed and Nelson, 1977) and is best interpreted as an olistolith within the flysch. Other known Triassic rocks elsewhere in the structurally composite McKinley terrane are

radiolarian cherts and pillow basalt and related mafic

intrusive rocks.

Farther east and on the south side of the Denali fault

Monotis is known from the Chulitna, Susitna, Nenana, and

Wrangellia terranes. In the Chulitna terrane poorly preserved Monotis, questionably identified as M. (P.) subcircularis (pl. 4, figs. 9–10), occurs at locality 17 (fig. 1), which

is map locality 34 of Reed and Nelson (1977). This is the

only occurrence known in the Chulitna terrane. The ageequivalent strata farther to the northeast in the terrane probably represent a more shallow-water facies (Jones and

others, 1980).

In the Susitna terrane Monotis is more abundant but

known only from float collections at locality 18 (fig. 1).

Consequently, although both Monotis (P.) ?subcircularis

and M. (M.) alaskana are represented (pl. 4, figs. 6–8), their

relative stratigraphic position is unknown. The entire Susitna terrane is a huge recumbent fold of pillow basalt and

intercalated volcaniclastic sedimentary rocks tectonically

enclosed in deformed Upper Jurassic and Cretaceous flysch

of the Kahiltna terrane. The Nenana terrane occurs nearby,

also in fault contact with Upper Jurassic and Cretaceous

flysch. It consists, however, mainly of metamorphosed

impure limestone and calcareous sandstone in which poorly

preserved Monotis that resemble M. (P.) subcircularis have

been found at one place (loc. 22, fig. 1).

In the Wrangellia terrane Monotis occurs at many localities in the McCarthy Formation, typical exposures of which

are in the southern Wrangell Mountains (MacKevett, 1978)

near locality 19 (fig. 1), but which is known as far west as

locality 23 (fig. 1), on the south flank of the Alaska Range

(Silberling and others, 1981). In its lower part, which is latest Triassic and Early Jurassic(?) in age, the McCarthy Formation is regularly interbedded calcareous carbonaceous

shale, impure limestone, and impure spiculitic chert (MacKevett, 1970). These deep-marine rocks were deposited

after the huge plateau of Triassic oceanic basalt that distinguishes Wrangellia foundered during Late Triassic time.

Pervasive minor structures make stratigraphic measurements within the McCarthy Formation inaccurate, but Monotis (pls. 3–4) occurs at various levels through at least 60 m of

section. The stratigraphically lowest occurrences of Monotis

are generally 50–100 m above the base of the McCarthy and

are characterized by a profusion of M. (P.) subcircularis,

with which a few specimens of M. (Eomonotis) jakutica are

sometimes associated. In the few places where its occurrence has been observed in bedrock, M. (M.) alaskana

invariably is stratigraphically above M. (P.) subcircularis.

Monotis (M.) alaskana has its type locality (USGS

Mesozoic loc. 9961) in the McCarthy Formation of the

Wrangellia terrane about 40 km west of the area in the

Wrangell Mountains from which the specimens illustrated

herein were collected. This species from the Wrangell

Mountains was regarded as a synonym of M. (M.) salinaria

by Silberling (as in MacKevett, 1970; Silberling and others,

1981) but, as explained previously, it is now regarded as

being distinct. The more coarsely ribbed specimens figured

as M. (M.) ?haueri (pl. 3, figs. 8–13) are the only ones of this

morphology from a measured section and occur 18 m above

M. (M.) alaskana.

SOUTHWESTERN ALASKA

In the Kuskokwim region of southwestern Alaska,

small lots of Monotis have been collected during the course

of geologic investigations from several different localities,

four of which are plotted as localities 25–28 in figure 1. Of

these, localities 26 (USGS Mesozoic locs. M12909 and

M18927) and 27 (USGS Mesozoic locs. 23170 and M6671)

represent the Togiak and Goodnews terranes, respectively,

whereas the terrane assignments of localities 25 and 28 are

less certain. The Togiak terrane is interpreted by Box (1985)

as a Late Triassic to Early Cretaceous intraoceanic volcanic

arc, against and beneath whose northwestern margin (in

present-day coordinates) the Goodnews terrane was structurally emplaced as a subduction complex. Locality 25 is from

an isolated exposure along the Kuskokwim River (USGS

Mesozoic loc. M1879) that might be an outlier representing

older strata of the Nyac terrane. The Nyac is another belt of

intraoceanic arc rocks from which Middle Jurassic fossils are

known. Original continuity along strike of the Nyac and

Togiak terranes prior to large-scale Cenozoic strike-slip

faulting is suggested by Box (1985). Locality 28 (USGS

Mesozoic locs. 17080 and D10708) is along the boundary

between the Togiak and Tikchik terranes. It is in rocks that

probably represent the older parts of the Togiak terrane, the

underlying Tikchik terrane being either part of a subduction

complex structurally below the Togiak or perhaps even part

of the original depositional substrate of the Togiak. Thus,

the Monotis occurrences in southwestern Alaska are probably representative of a single, major, heterogeneous Mesozoic intraoceanic arc and its accretionary subduction

complex, all of which were amalgamated by Early Cretaceous time (Box, 1985). Most of the meager and poorly preserved Monotis faunas in collections from these rocks, at

least provisionally, can be assigned to the ubiquitous M. (P.)

subcircularis, along with a few specimens apparently having

the morphology of M. (En.) o. densistriata and M. (M.)

haueri. As explained previously, the morphometric limits of

these two species may include, respectively, inflated immature and densely ribbed variants of M. (P.) subcircularis.

One Monotis collection (USGS Mesozoic loc. M6671) from

the belt of outcrops represented by locality 27 (fig. 1)

deserves special attention, however, because it contains only

conspicuously inflated, small- to medium-size shells, preserved as fragmentary molds. In the absence of associated
11

fragments of large, M. (P.) subcircularis-like shells, this

occurrence may represent a population assignable to a midor high-paleolatitude representative of M. (Entomonotis),

such as M. (En.) ochotica, its ribbing being coarser than that

of the paleolatitudinally ambiguous M. (En.) densistriata.

In the Peninsular terrane, Monotis is especially well

represented from a number of localities in the vicinity of

Cape Kekurnoi (loc. 21, fig. 1) on the east side of the Alaska

Peninsula (pl. 2). It also has been collected farther north in

the Peninsular terrane on either side of the mouth of Cook

Inlet, collections from the east side of the inlet having been

obtained from Port Graham (loc. 29, fig. 1).

At locality 21, in wave-cut exposures along the shores

of Puale and Alinchak Bays, on either side of Cape

Kekurnoi, several hundred meters of Upper Triassic,

well-bedded, dense limestone, shale, and minor volcaniclastic rocks (the “bedded cherts” lithofacies of Wang and others, 1988) overlie platform-carbonate and volcanic rocks, all

of which are late Norian in age. Monotis (P.) subcircularis

is well represented in collections from relatively low in the

well-bedded sequence, however, M. (M.) alaskana occurs

near its top. Although the section is disturbed by faults and

folds, Detterman and others (in press) reported a minimum

stratigraphic separation of about 75 m between these two

species in an unbroken partial section in Alinchak Bay.

Monotis (P.) subcircularis and M. (M.) alaskana are

also represented in different collections (for example,

respectively, USGS Mesozoic loc. 7234 and 10546) from

dense, black carbonaceous limestone of the Peninsular terrane from Port Graham (loc. 29, fig. 1). Stratigraphic relationships of the collections from this area are not known, but

in rocks of the same character as those in which the late

Norian fossils occur, the characteristic middle Norian flat

clams Halobia lineata and H. dilitata occur in separate collections (respectively, USGS Mesozoic locs. 6380 and 6382)

as does the late middle Norian ammonite Steinmannites

(from USGS Mesozoic loc. D13388). Thus, unlike the Peninsular terrane at locality 21 (fig. 1), where the evenly bedded pelagic strata containing flat clams are entirely late

Norian, strata of this kind are of both middle Norian and late

Norian age at locality 29. This is significant because,

although M. (M.) alaskana is nowhere known in stratigraphic sequence with any of the characteristically middle

Norian species of M. (Eomonotis), at most localities, if

present at all, middle Norian strata are of different facies than

the Monotis-bearing upper Norian strata. At Port Graham,

where late Norian M. (M.) alaskana occurs, middle Norian

strata of the flat clam-ammonite facies are apparently well

represented, and yet Monotis is not found within them.

SOUTHEASTERN ALASKA

Monotis occurs in the Alexander terrane at localities in

Keku Strait (Muffler, 1967) (loc. 20, fig. 1) between Kuiu

and Kupreanof Islands, and farther to the north on Admiralty

Island. In Keku Strait, M. (P.) subcircularis is represented

in collections from the Hound Island Volcanics on either

side of the strait; on the west side it occurs in a remarkably

fossiliferous bed of partly silicified marl about 1 m thick

(USGS Mesozoic loc. M1912). In this bed, this species is

associated with M. (En). cf. M. (En.) ochotica densistriata

(pl. 1), which, as explained previously, might be no more

than inflated immature variants of M. (P.) subcircularis.

Additionally, from this same bed there are as many as 50

other kinds of marine invertebrates, mostly bivalves.

Among the bivalves is Septocardia cf. S. pascoensis, previously reported only from Peru (Newton, 1983). From other

localities on the islands within Keku Strait, the subgenus

Eomonotis occurs sparsely with middle Norian halobias, as

for example at USGS loc. M1900 where M. (Eo.) ?pinensis

occurs with Halobia cf. H. fallax (pl. 1, figs. 15–16). As

explained below, M. (Eomonotis) generally indicates middle

to high paleolatitudes; thus, the paleobiogeographic interpretation is in agreement with the original paleomagnetic results

from the Hound Island Volcanics by Hillhouse and Grommé

(1980) who found the paleolatitude of these volcanic rocks

of the Alexander terrane to be not significantly different

from that of the adjacent margin of cratonic North America.

This fits the biogeographic data, because the present-day latitude of Keku Strait is approximately that of the Peace River

Foothills of British Columbia, the southernmost occurrence

of M. (Eo.) pinensis on the craton. To circumvent the platemotion traffic-pattern problem of having an early Mesozoic

mid-latitude Alexander terrane parked outboard of relatively

low-latitude early Mesozoic terranes, such as Stikinia, the

possibility of a southern, rather than northern, paleolatitude

for the Alexander terrane was suggested (Newton, 1983;

Panuska and Stone, 1985; Silberling, 1985).

Subsequent studies contradict these original interpretations regarding the paleolatitude of the Alexander terrane.

First, according to Gardner and others (1988), a Pennsylvanian granitic complex, which is extensive within the Alexander terrane, crosscuts the contact between the Alexander

and Wrangellia terranes in southern Alaska near the International Border. This implies that the two terranes have travelled together at least since the Pennsylvanian. As the

Triassic paleolatitude of Wrangellia is well established in the

range of 10°–17° (Hillhouse and Grommé, 1984) , that of an

adjoined part of the Alexander terrane should also be low.

Second, restudy of the paleomagnetism of the Hound Island

Volcanics by Haeussler and Coe (1992) indicates that the

magnetic direction originally reported by Hillhouse and

Grommé (1980) was a fortuitously realistic combination of

two magnetic overprint directions and that the primary component of magnetization in these rocks yields a paleolatitude

of 19.2°+10.3°, in much better agreement with the paleolatitude of Wrangellia. A reappraisal of all the collections of

fossils from Triassic rocks of the Alexander terrane also

yields some biogeographic support for this current viewpoint. For example, a fairly diverse late Norian shelly fauna

OCCURRENCE OF MONOTIS IN ALASKA
12 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

from Gravina Island near the southern terminus of the

Alexander terrane, about 225 km south-southeast of locality

20 (fig. 1), is most similar to paleoequatorial faunas from the

Methow-Tyaughton terrane of southern British Columbia

and of the Gabbs Formation of the Paradise terrane in western Nevada (N.J. Silberling, unpub. data). Among several

collections of Monotis (M.) subcircularis from Mole Harbor,

in the Alexander terrane on the east side of Admiralty Island

about 90 km north of locality 20 (fig. 1), one collection

(USGS Mesozoic loc. 10168) can be assigned to M. (M.)

alaskana, a Monotis species typical of Wrangellia and a

biogeographic hallmark of low paleolatitudes.

Both mid- and low-paleolatitude biogeographic indices, thus, occur among the faunas of the Alexander terrane.

They are, however, from different exposures than those that

yielded the current understanding of the original component

of paleomagnetism in the Hound Island Volcanics (Haessler

and Coe, 1992) in the vicinity of Keku Strait and those from

which shells of fossils of apparent South American affinity

were collected. Structural deformation of the Triassic rocks

in this area is intense enough to support suspicion that different facies are juxtaposed on as yet unrecognized faults.

However, the amount of post-Triassic tectonic juxtaposition

within Keku Strait itself is limited, because the boundary of

major juxtaposition of Paleozoic facies that passes through

Keku Strait and that separates the Craig and Admiralty subterranes of the Alexander terrane, is overlapped by the Pybus

Formation of Permian age (Berg and others, 1978).

In an attempt to rationalize all the current geologic and

paleontologic data from the Alexander terrane, it can be

pointed out that the terrane, as presently configured, subtends about 10° of latitude. Moreover, post-Triassic

structural disruption within the Alexander terrane could have

been sufficient to have imbricated or telescoped an even

greater original latitudinal span. Thus, paleolatitudes

approximating a 15°–30° range within Alexander terrane

rocks might be permissible and would satisfy the paleolatitudinal perceptions based on the relevant Monotis faunas. In

view of the right-slip Mesozoic and Cenozoic history on

major faults that are subparallel to this part of the Pacific

margin and the relatively outboard position within the Alexander terrane of Keku Strait, location of the Alexander terrane in relatively low latitudes of the southern hemisphere

during the Late Triassic is still an attractive hypothesis.

AGE AND BIOSTRATIGRAPHY OF 

ALASKAN MONOTIS FAUNAS

Monotis-bearing strata are broadly correlated with the

middle and upper Norian Stage of the Upper Triassic (for

example, Tozer, 1980). In more recent studies (GrantMackie, 1986; Grant-Mackie and Silberling, 1990), beds

containing species of M. (Eomonotis) are mainly correlated

with the Columbianus Zone, the ammonite zone that forms

the upper part of the middle Norian. Beds containing species

of the subgenera M. (Monotis), M. (Pacimonotis), and M.

(Entomonotis) are correlated with the Cordilleranus Zone

that defines the lower part of the upper Norian. Monotis

(Eo.) jakutica is the only one of the known Alaskan species

of the subgenus Eomonotis that occurs in the upper Norian

Cordilleranus Zone. In Alaska and elsewhere, there is no

compelling evidence that any Monotis species appears either

in strata below the Columbianus Zone or higher than the

Cordilleranus Zone (Tozer, 1980).

Aside from the occasional occurrence of ammonoids in

Monotis-bearing strata of Alaska, other kinds of marine

megafossils associated with Monotis are mainly restricted to

the morphologically distinct flat clam Halobia and the enigmatic hydrozoan Heterastridium. Halobia, various species

of which are characteristic of Karnian through middle

Norian strata, co-occurs with different species of M.

(Eomonotis) at a number of places in Alaska, commonly on

the same bedding surfaces. It is found, for example, with M.

(Eo.) daonellaeformis in the Upper Yukon, with M. (Eo.)

typica in the Endicott Mountains and North Slope subterranes, with M. (Eo.) obtusicostata and M. (Eo.) ?jakutica in

the North Slope subterrane, and with possible or probable M.

(Eo.) pinensis in the North Slope subterrane and the Alexander and McKinley terranes. All these occurrences are in

the late middle Norian; nowhere in North America is Halobia known to extend stratigraphically above the Columbianus Zone.

Heterastridium, the other common associate of Monotis, was evidently a planktonic organism that is locally abundant in the Nixon Fork, Togiak(?), Peninsular, Wrangellia,

and Alexander terranes. These enigmatic fossils are spherical, unless compacted into oblate-spheroids, commonly from

10 to more than 50 mm in diameter, contain a delicate, radial,

cellular interior meshwork, and have a distinctive outer surface, densely covered by low, isolated papillae. Heterastridium is known only from the Cordilleranus Zone in North

America, but elsewhere in the world it has been reported

from older Norian strata.

Although much of the present understanding of the biostratigraphic distribution of Monotis is based on Alaskan

successions (in addition to those in Canada, Far Eastern Russia, Japan, and New Zealand), there are few welldocumented, stratigraphically unbroken Alaskan sections

from which the sequential relations among the various species can be established. The most informative sequential

data from Alaskan occurrences of Monotis (in terms of

USGS Mesozoic localities) are as follows:

(1) In the Upper Yukon, M. (Eo. daonellaeformis (loc.

M1714; pl. 6, figs. 1–5) occurs 3–10 m below M.

(Eo.) ?pinensis (loc. M1715; pl. 6, figs. 6–16). This

is the only unquestioned occurrence of M. (Eo.)

daonellaeformis in Alaska, but in northeastern Asia,

Dagys and others (1979) define a M. daonellaeformis
13

subzone of the “M. scutiformis zone,” overlain by a

M. pinensis subzone.

(2) In the Simpson No. 1 test well in the North Slope subterrane, M. (Eo.) typica (as figured on plate 11)

occurs in the core segment from 6,300–6,306 ft,

above the occurrence of M. (Eo.) pinensis in the core

segment at 6,307–6,318 ft. This is the reverse of the

stratigraphic order reported from Far Eastern Russia

by Arkhipov (1970).

(3) In the eastern part of the North Slope subterrane from

the surface reference section of the Shublik Formation and the overlying Karen Creek Sandstone at

Fire Creek, three of the Monotis species figured on

plate 11 occur in sequence. Here, M. (En.) ochotica,

sensu stricto, in the basal part of the Karen Creek,

overlies M. cf. M. (Eo.) pinensis, which is underlain

about 10 m lower by M. (Eo.) obtusicostata.

(4) One of the most complete successions of different

kinds of Monotis was collected by I.L. Tailleur

(U.S. Geological Survey) from the almost completely exposed, subvertical, condensed section of

the Otuk Formation along the Kukpuk River at loc.

M2156 in a Triassic section like that of the Endicott

Mountains subterrane. As measured from highest

to lowest, beneath the apparent top of the Otuk, the

following forms are represented:

6.7–7.3 m ......................... M.(En.) ochotica ochotica

 8.2 m .......................... M. (En.) ?pachypleura

 10.0 m ..........................M. (M.) sp. aff. (P.)

 subcircularis and M. (Eo.) sp. cf. jakutica

 11.3 m ..........................M. (Eo) ?jakutica

 12.2 m ..........................M. (Eo.) sp. cf. typica

 13.7 m .......................... M. (Eo.) ?typica and 

 M. (Eo.) sp. aff. obtusicostata

 14.6 m ......................... M. (Eo.) sp. indet. and

Halobia sp. indet.

Unfortunately, most of these collections are not

well preserved, so most identifications are uncertain, but sequential relations of the various forms

accord well with those found elsewhere.

(5) Monotis (Eo.) obtusicostata was originally described

by Westermann (1966) from British Columbia in

association with M. (Eo.) pinensis. Specimens provisionally referred to M. (Eo.) obtusicostata (Dutro

and Silberling, 1988, pl. 30.1, figs. 6–7) and M.

(Eo.) pinensis (pl. 6, fig. 12–16), respectively from

the North Slope subterrane and the Upper Yukon

region, are on the same bedding surfaces with the

ammonite Neohimavatites cf. N. canadensis

(McLearn), which is indicative of the uppermost of

the three subzones established by Tozer (1980)

within the Columbianus Zone.

(6) In the Nixon Fork terrane, M. (Eo.) anjuensis (pl. 5)

was collected from a single bed (loc. D11286),

stratigraphically above which M. (P.) subcircularis

was collected in float. At another nearby section,

float specimens in different slabs of questionable

M. (Eo.) typica and M. (Eo.) jakutica occur 4–5 m

below the occurrence of M. (P.) subcircularis in

bedrock.

(7) Monotis (En.) ochotica (sensu stricto) occurs 2.4 to 4.6

m above M. (P.) subcircularis at locs. M2650

through M2652 from Fire Creek in the North Slope

subterrane in collections other than those containing the specimens figured on plate 11. This superposition is probably also true in the Endicott

Mountains subterrane, as noted in (4). In northeastern Asia, however, M. (En.) ochotica apparently

occurs earlier, immediately above beds containing

M. (Eo.) pinensis and M. (Eo.) scutiformis, and

characterizes the younger of two Monotis zones

recognized there (Dagys and others, 1979;

Arkhipov and Truschelev, 1982; Bychkov and Polubotko, 1984). This may explain the apparent

occurrence of M. (En.) ochotica below M. cf. M.

(P.) subcircularis in the West Dease No. 1 well,

North Slope terrane (pl. 11, figs. 13–15). In British

Columbia, these two species are reported by Westermann and Verma (1967) to occur together.

Nowhere is M. (En.) ochotica found in sequence

with M. (M.) alaskana.

(8) Monotis (Eo.) jakutica is known from Wrangellia, in

association with M. (P.) subcircularis (pl. 3, figs.

16–20), from the Nixon Fork terrane with M. (Eo.)

?typica (pl. 5, figs. 5–10), and possibly from the

North Slope subterrane with M. (Eo.) obtusicostata

(pl. 10, figs. 7–12). Near Point Hope, M. (Eo.)

jakutica occurs with poorly preserved, but probable, M. (P.) subcircularis at locs. M2546 and

M2541 in the Otuk Formation; from the same general area, it occurs with definite M. (P.) subcircularis at loc. M2545; and in British Columbia these

two species are recorded together by Westermann

and Verma (1967). Various authors (for example,

Arkhipov and Truschelev, 1982) have stated that in

northeastern Asia M. jakutica is found through a

sequence having M. (Eo.) scutiformis in its lower

part and M. (En.) ochotica (s.s.), M. (En.) o. densistriata, M. (En.) pachypleura, and M. (P.) subcircularis in its upper part. Thus, M. (Eo.) jakutica

apparently ranges through a greater part of the

Monotis sequence than most other species, and it

occurs in both the Columbianus and Cordilleranus

ammonite zones; however, in Alaska it is most

commonly found in strata containing M. (P.)

subcircularis.

(9) Monotis (En.) ochotica densistriata has been recorded

from British Columbia in association with M. (P.)

subcircularis (for example, by Westermann and

Verma, 1967), and specimens similar to this

AGE AND BIOSTRATIGRAPHY OF ALASKAN MONOTIS MNOTIS FAUNAS
14 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

subspecies (pl. 1, figs. 1–3, 11–12) occur with definite M. (P.) subcircularis in the Alexander terrane.

In parts of Asia this subspecies is taken to be

characteristic of the lower part of the range of M.

(Entomonotis). For example, (1) it characterizes

the Densistriata zone of Nakazawa (1964) in this

stratigraphic position in Japan; (2) along with M.

zabaikalica and M. (Eo.) jakutica, it characterizes

the Zabaikalica subzone of Dagys and others

(1979), which forms the lower part of their Ochotica zone in eastern Russia; and (3) again in eastern

Russia, it is reported by Arkhipov and Truschelev

(1982) as characteristic of beds containing M. (En.)

ochotica below those with M. (P.) subcircularis.

(10) Monotis (En.) pachypleura occurs in isolated collections from the more western exposures of the

Endicott Mountains subterrane. In addition to the

occurrences figured on plate 9, at loc. M2157, in

another nearby section of the Otuk Formation, it

occurs 1.8 m above beds with fragmentary Monotis

identified as M. sp. aff. (P.) subcircularis and M. cf.

(Eo.) jakutica, and 0.9 m below strata with M. (En.)

ochotica (s.s.). At this locality M. (En.) pachypleura is associated with fragments of a large indeterminate Monotis having coarse, rounded ribs,

such as those of M. (En.) ochotica.

(11) Monotis (M.) alaskana occurs exclusively at a higher

level than M. (P.) subcircularis in both the Peninsular and Wrangellia terranes, as illustrated by the

examples figured on plates 2 and 3. The other species of M. (Monotis) in Alaska, M. (M.) haueri, is

probably represented by specimens (pl. 3, figs.

8–13) still higher in the Wrangellia-terrane section,

above the occurrence of M. (M.) alaskana. M. (M.)

haueri as identified from other terranes may be

appreciably older and an associate, perhaps even a

variant of M. (P.) subcircularis.

Summarizing the itemized relationships, the gross

stratigraphic ranges of the various kinds of Monotis known

from Alaska are shown in figure 3. Although the actual

ranges and the phylogenetic relationships among different

forms are far from being established, some generalizations

are, nevertheless, possible about the morphologic evolution

within the genus. Obvious evolutionary trends are (1) for the

posterior ear to lose its ribbed ornamentation and become

smooth and more distinctly separated in outline from the rest

of the shell; (2) for an increase in maximum relative size; and

(3) at least among the earlier forms, for the ribbing to

become better differentiated into different orders. Among

younger forms, in different lineages there are tendencies

towards broader ribs, fading of the ribbing, or both, and

towards development of a strongly inflated shape. Despite

some overlap in ranges, species of M. (Eomonotis) appear

first and somehow are ancestral to those of M. (Pacimonotis), M. (Monotis), and M. (Entomonotis); the last two of

these subgenera may include the youngest representatives of

the genus in North America.

PALEOGEOGRAPHIC SIGNIFICANCE 

OF ALASKAN MONOTIS FAUNAS

The known occurrences of the various forms of Monotis

in the accreted terranes of Alaska and in the non-accretionary

North American rocks in the Upper Yukon region of Alaska

are tabulated in figure 4, which shows that different kinds of

Monotis occur in Alaskan rocks at different places. The

paleogeographic significance of this distribution requires

consideration of the paleoecologic constraints on the distribution of these organisms that do not have close morphologic counterparts among modern bivalves. Although not

universally accepted (Kauffman, 1988), the widely held

view is that most Monotis had a pseudoplanktonic mode of

existence, living attached to floating objects, such as sea

weeds (for example, Tozer, 1982; Gall, 1983). Evidence for

this is (1) the remarkable thinness of their shells compared to

their all-over size; (2) their wide distribution in open-marine

deposits, apparently without regard to the nature of the substrate; (3) the nature of the inconspicuous anterior ear, which

forms a pseudoctenolium indicative of shell attachment to

some object by byssal filaments (Grant-Mackie and Silberling, 1990); and (4) the concave-upward preferred orientation of inflated valves of New Zealand examples in pelagic

limestones resting on pillow basalts, indicating descent of

the shells through the water column unaffected by lateral

currents (Silberling and others, 1988). This inferred pseudoplanktonic, near-surface mode of life implies that water temperature, and thus paleolatitude, should be a primary factor

in controlling the north-south distribution of different kinds

of Monotis.

The unique nature of the Pacific and Arctic margins of

North America in displaying a record of the marine Triassic

across paleolatitudes ranging from paleoequatorial to moderately high was first discussed by Tozer (1982). Inboard of

the Phanerozoic accreted terranes, exposures of cratonbound, or at least craton-related, fossiliferous marine Triassic rocks from western Nevada, the Rocky Mountain Foothills of western Canada, and the Canadian Arctic Islands

allow comparisons between, respectively, low, low-middle,

and high-middle paleolatitude faunas, including Late Triassic Monotis faunas (Silberling, 1985). Differences among

currently cited Norian North American paleomagnetic poles

(Haeussler and Coe, 1992; Kent and Witte, 1993), even those

from the Colorado Plateau that differ widely from other

Norian poles, generally scatter along a trace parallel to the

pre-Jurassic North American apparent polar wander path

that is roughly normal to the trend of the North American

Pacific margin. Thus, paleolatitudes of Norian localities

along the cratonic margin are not much affected by these
15

M. (P.) subcircularis M. (En.) o. densistriata

M. (En.) pachypleura

M. (M.) alaskana

M. (M.) haueri

M. (En.) ochotica (s.s.)

M. (Eo.) jakutica

M. (Eo.) pinensis

M. (Eo.) daonellaeformis

M. (Eo.) obtusicostata

M. (Eo.) anjuensis

M. (Eo.) typica

?

?

?

?

?

? ?

?

COLUMBIANUS ZONE (part)

CORDILLERANUS ZONE (part?)

N O R I A N 

Upper (part) Middle (part)

(part)

Figure 3. Chart showing the relative stratigraphic ranges of Monotis species and subspecies represented in Alaska.

differences in pole position that vary as much as 30° in angular spread. In terms of the revised taxonomic nomenclature

used herein, faunal lists compiled from the literature for

North American craton-bound Monotis faunas of broadly

different paleolatitudes are as follows:

Nevada (low paleolatitude; about 10° N.)

Monotis (Monotis) alaskana

M. (M.) haueri

M. (Pacimonotis) subcircularis

Northeastern British Columbia (low-middle paleolatitude; about 25°–30° N.)

M. (Eomonotis) callazonensis

M. (Eo.) jakutica

M. (Eo.) obtusicostata

M. (Eo.) pinensis

M. (Eo.) typica

M. (Entomonotis) ochotica s.s.

M. (En.) o. posteroplana

Monotis (Pacimonotis) subcircularis

Canadian Arctic Islands (high-middle paleolatitude;

about 50° N.)

M. (Eomonotis) daonellaeformis

M. (Eo.) jakutica

M. (Eo.) typica

M. (En.) o. posteroplana

Monotis (Pacimonotis) subcircularis

In addition to the implied paleolatitudinal control on the

distribution of different forms of Monotis, differences

among the faunas from different segments of the cratonic

margin could also result from such factors as collection bias

and the possibility that strata of appropriate facies and age

may not be represented in some regions. However, the

mutual exclusion in faunas of different paleolatitudes of M.

(Monotis) alaskana on the one hand from nearly all species

of M. (Eomonotis) and M. (Entomonotis) on the other is

regarded as significant. In support of this, in western Nevada

the late Norian Monotis (Pacimonotis) subcircularis occurs

widely, in places in the same sections containing M. (M.)

alaskana, in the Jungo terrane that is related depositionally

to low-latitude cratonic North America (Lupe and Silberling,

1985). In older parts of the Jungo terrane, middle Norian,

including late middle Norian, strata are thick and well dated

by ammonite and Halobia faunas, and yet no trace of an

Eomonotis has ever been found. Similarly, in the northern

part of the Stikinia terrane in the southern Yukon, Canada

(loc. 24, fig. 1), only M. (P.) subcircularis and M. (M.) cf.

haueri are known to occur (Westermann, 1966); however,

species of M. (Entomonotis), such as those that characterize

craton-bound late Norian Monotis faunas farther inland at

about the same present-day latitude, are unknown. Currently

accepted North American Triassic and Jurassic paleomagnetic pole positions place North America at a lower paleolatitude than that previously accepted. Some geologists, for

example, Nelson and Mihalynuk (1993), therefore, discount

the importance of latitudinal shift of the Stikinia terrane.

Still, an appreciable northward displacement of Stikinia with

respect to its Triassic record still seems required by the

paleobiogeographic relationships. For example, in the same

stratigraphic succession as that containing the Monotis faunas at locality 24 (fig. 1) in the Stikinia terrane, one of the

PALEOGEOGRAPHIC SIGNIFICANCE OF ALASKAN MONOTIS FAUNAS
16 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

North Slope

 (incl. AAN-AAE)

Endicott Mts. (and

 De Long Mts.)

North America

 (Upper Yukon region)

Nixon Fork

McKinley

Goodnews

Nyac?

Togiak

Chulitna

Susitna

Nenana

Stikinia

Wrangellia

Peninsular

Alexander

M. (En.) o. densistriata

Monotis (M.) alaskana

M. (M.) haueri

M. (P.) subcircularis

M. (En.) pachypleura

M. (En.) ochotica s.s.

M. (Eo.) anjuensis

M. (Eo.) jakutica

M. (Eo.) pinensis

M. (Eo.) typica

M. (Eo.) obtusicostata

x X XXX

X X X

X

X x

X X x x

x

x

x

X x

x

X

XX X

X X

x X x x

TERRANES

M. (Eo.) daonellaeformis

(and subterranes)

X

x

aff.

x X

X

x x

xx x

x

x

only two developments of true paleoequatorial Dachsteintype Upper Triassic coral reefs in North America (Stanley

and Senowbari-Daryan, 1986)) is displayed, the other being

in the Wallowa terrane of eastern Oregon. The implications

of this for appreciable northward displacement of the Stikinia

terrane relative to the craton was first recognized by Tozer

(1970).

The paleogeographic meaning of the distribution of the

various kinds of Monotis in the accreted terranes of Alaska is

simple and obviously related to major geologic features.

Low-paleolatitude faunas characterize terranes that lie to the

south of the Denali fault and have been transported northward relative to North America. Monotis faunas suggest that

low paleolatitude terranes south of the Denali fault, such as

the Wrangellia and Susitna terranes, are structurally juxtaposed against the Nixon Fork terrane on the north side of the

fault. The Nixon Fork contains an unusual association of

Monotis species, generally of mid-paleolatitude kind, but as

noted previously it also contains the northernmost known

occurrence of Heterastridum; this terrane thus has biogeographic characteristics intermediate between those of the

low-paleolatitude terranes of southern Alaska and the midpaleolatitude terranes of northern Alaska. The Monotis

fauna of the small Upper Yukon region in Alaska, which is

regarded as a non-accretionary part of Phanerozoic North

America, resembles that of mid-paleolatitude kind. Northward displacement of Stikinia with respect to the Upper

Yukon was accommodated by slip on the Tintina fault–

Northern Rocky Mountain Trench (fig. 1).

The Monotis-bearing terranes and the Upper Yukon

region are listed in order from north to south in figure 4 in

their approximate relative paleolatitudinal and hemispheric

positions as suggested by the biogeographic affinities of

their Monotis and other faunas. Aside from hemispheric

ambiguity in placement of paleoequatorial terranes, no significant conflict arises between this pattern and the results of

paleomagnetic investigations.

REFERENCES CITED

Arkhipov, Yu.V., l970, New zonal scheme for the Upper Triassic of

the Yana River Basin: Doklady Akademii Nauk SSSR, v. 195,

p. 151–154.

Arkhipov, Yu.V. and Truschelev, A.M., l982, On biostratigraphy of

Norian-Rhaetian deposits of north-eastern Asia: Akademiya

Nauk SSSR, Sibirskoye Otdeleniye, Trudy Instituta

Geologogii i Geofiziki, v. 462, p. 48–54.

Berg, H.C., Jones, D.L., and Coney, P.J., 1978, Map showing preCenozoic tectonostratigraphic terranes of Southeastern Alaska

and adjacent areas: U.S. Geological Survey Open-File Report

78–1085, scale 1:1,000,000.

Blome, C.D., Reed, K.M., and Tailleur, I.L., 1988, Radiolarian biostratigraphy of the Otuk Formation in and near the National

Petroleum Reserve, northern Alaska, in Gryc, George, ed.,

Geology and exploration of the National Petroleum Reserve in

Alaska, 1974 to 1982: U.S. Geological Survey Professional

Paper 1399, p. 725–776.

Bodnar, D.A., 1984, Stratigraphy, age, depositional environments,

and hydrocarbon source rock evaluation of the Otuk Formation, North-Central Brooks Range, Alaska: M.S. thesis,

University of Alaska, Fairbanks, 232 p.

Box, S.E., 1985, Terrane analysis of the northern Bristol Bay

region, southwestern Alaska: U.S. Geological Survey Circular

967, p. 32–37.

Brabb, E.E., 1969, Six new Paleozoic and Mesozoic formations in

east-central Alaska: U.S. Geological Survey Bulletin 1274-I,

26 p.

Figure 4. Checklist showing occurrence of different Monotis

species in the accreted terranes of Alaska, the non-accretionary

North American rocks of the Upper Yukon, and the Stikinia terrane

of western Canada. As listed from top to bottom, terranes are in order from north to south in their inferred approximate relative

paleolatitudinal and hemispheric positions. X, species regarded as

present without qualification; x, probably or possibly present (in

the sense of “cf.” or “?”).

 
17

Bychkov, Yu.M., and Polubotko, I.V., 1984, Upper Triassic, in Rostovstev, K. O., and Zhamoida, A. J., ed., A standard for Triassic

time in the USSR: Leningrad, Ministry of Geology and USSR

Academy of Sciences (in Russian), p. 63–104.

Dagys, A.S., Arkhipov, Yu.V., Bychkov, Yu.M., 1979, Stratigraphy

of the Triassic System of North-Eastern Asia: Akademiya Nauk

SSSR, Sibirskoye Otdeleniye, Trudy Instituta Geologogii i

Geofiziki, v. 447, 243 p.

Detterman, R.L., Case, J.E., Miller, J.W., Wilson, F.H., and Yount,

M.E., in press, Stratigraphic framework of the Alaska Peninsula: U.S. Geological Survey Bulletin 1969–A.

Detterman, R.L., Reiser, H.N., Brosge, W.P., and Dutro, J.T., Jr.,

1975, Post-Carboniferous stratigraphy, Northeastern Alaska:

U.S. Geological Survey Professional Paper 886, 46 p.

Dutro, J.T., Jr., and Silberling, N.J., 1988, Megafossil biostratigraphy of some deep test wells, National Petroleum Reserve in

Alaska, in Gryc, George, ed., Geology and exploration of the

National Petroleum Reserve in Alaska, 1974 to 1982: U.S. Geological Survey Professional Paper 1399, p. 667–675, pls.

30.1–30.4.

Gall, J.C., 1983, Ancient sedimentary environments and habitats of

living organisms: Berlin, Springer-Verlag, 219 p.

Gardner, M.C., Bergman, S.C., Cushing, G.W., Mackevett, E.M.,

Jr., Plafker, G., Campbell, R.B., Dodds, C.J., McClelland,

W.C., and Mueller, P.A., 1988, Pennsylvanian pluton stitching

of Wrangellia and the Alexander terrane, Wrangell Mountains,

Alaska: Geology, v. 16, p. 967–971.

Grant-Mackie, J.A., 1981, New Zealand Warepan (Upper Triassic)

sequences: Murihiku Supergroup of the North Island: Royal

Society of New Zealand Journal, v. 11, p. 31-56.

Grant-Mackie, J.A., 1986, The Warepan Stage of the Upper Triassic: Redefinition and subdivision: New Zealand Journal of

Geology and Geophysics, v. 28, p. 701–724.

Grant-Mackie, J.A., and Silberling, N.J., 1990, New data on the

Upper Triassic bivalve Monotis in North America and the new

Subgenus Pacimonotis: Journal of Paleontology, v. 64, p.

240–254.

Haeussler, Peter, and Coe, R.S., 1992, Paleomagnetism of the Late

Triassic Hound Island Volcanics: Revisited: Journal of

Geophysical Research, v. 97, no. B13, p. 9617–9639.

Hillhouse, J.W., and Grommé, C.S., 1980, Paleomagnetism of the

Triassic Hound Island Volcanics, Alexander Terrane, Southeastern Alaska: Journal of Geophysical Research, v. 85, no. B5,

p. 2594–2602.

———1984, Northward displacement and accretion of Wrangellia:

New paleomagnetic evidence from Alaska: Journal of

Geophysical Research, v. 89, no. B6, p. 4461–4477.

Jones, D.L., Silberling, N.J., and Coney, P.J., 1986, Collision tectonics in the Cordillera of western North America: Examples

from Alaska, in Coward, M.P., and Ries, A.C., eds., Collision

tectonics: Geological Society Special Publication 19, p.

367–387.

Jones, D. L., Silberling, N. J., Coney, P. J., and Plafker, George,

1987, Lithotectonic terrane map of Alaska (west of the 141st

Meridian): U.S. Geological Survey Miscellaneous Field

Studies Map MF–1874–A, scale 1:2,500,000.

Jones, D.L., Silberling, N.J., Csejtey, Béla, Jr., Nelson, W.H., and

Blome, C.D., 1980, Age and structural significance of ophiolite

and adjoining rocks in the Upper Chulitna district, south-central

Alaska: U.S. Geological Survey Professional Paper 1121–A,

21 p.

Kauffman, E.C., 1988, The case of the missing community: lowoxygen adapted Paleozoic and Mesozoic bivalves (“flat

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Kent, D.V., and Witte, W.K., 1993, Slow apparent polar wander for

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rotation: Tectonics, v. 12, p. 291–300.

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Lupe, Robert, and Silberling, N.J., 1985, Genetic relationship

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MacKevett, E.M., Jr., 1970, Geologic map of the McCarthy C-5

quadrangle, Alaska: U.S. Geological Survey GQ–899, scale

1:63,360.

——— 1978, Geologic map of the McCarthy quadrangle, Alaska:

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Monger, J.W.H., and Berg, H.C., 1987, Lithotectonic terrane map of

western Canada and southeastern Alaska: U.S. Geological Survey Miscellaneous Field Studies, Map MF–1874–B, scale

1:2,500,000.

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18 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

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Monotis: Journal of Paleontology, v. 41, p. 798–803.

APPENDIX 1. LOCALITY 

DESCRIPTIONS

U.S. Geological Survey (USGS) Mesozoic locality

numbers without a letter prefix are recorded in the Reston,

Virginia, laboratory; those having the prefixes “D” and “M”

are recorded, respectively, at the Denver, Colorado, and

Menlo Park, California, laboratories of the U.S. Geological

Survey. The only Stanford University locality number is

prefixed by “SU.” All of the collections to which these

locality numbers pertain are currently housed in the reference collections of the U.S. Geological Survey, Denver, Colorado. Lithotectonic (also called tectonostratigraphic)

terrane designations are from Jones and others (1987) and

Monger and Berg (1987).

USGS Mesozoic loc. 6380. Kenai Peninsula, southern

Alaska; north side of Port Graham; lat 59°21'08" N., long

151°48' W. Coll.: U.S. Grant and party, 1909, field No.

G662. [Seldovia 1:250,000 quadrangle; Peninsular terrane]

USGS Mesozoic loc. 6382. Kenai Peninsula, southern

Alaska; south side of Port Graham; lat 59°21'02" N., long

151°50'24" W. Coll.: U.S. Grant and party, 1909, field No.

G673. [Seldovia 1:250,000 quadrangle; Peninsular terrane]

USGS Mesozoic loc. 7234. Kenai Peninsula, southern

Alaska; south shore of Port Graham, 1 and 2/5 miles below

wharf. Coll.: G.C. Martin, June 28, 1911, field No. Station

61. [Seldovia 1:250,000 quadrangle; Peninsular terrane]

USGS Mesozoic loc. 9382. Yukon River, central

Alaska; south bank, southwest of Nation. Coll.: Eliot Blackwelder, June 20, 1915, field No. 1507. [Charley River

1:250,000 quadrangle; in rocks not included among the

Phanerozoic accretionary terranes]

USGS Mesozoic loc. 9961. Wrangell Mountains,

southern Alaska; Mill Creek tributary to Lakina River on

west side near forks of creek, 21

/2 miles from Lakina River;

from float in gulch below cliff. Coll.: F.H. Moffit, field No.

43, Sept. 3, 1916. [McCarthy 1:250,000 quadrangle;

Wrangellia terrane]

USGS Mesozoic loc. 10546. Cook Inlet, Kenai Peninsula, southern Alaska; northerly shore of Port Graham near

mouth of small stream northeast from cannery, south shore

of Kachemak Bay. Coll.: A.C. Gill, July 20, 1918, field No.

18–G–90. [Seldovia 1:250,000 quadrangle; Peninsular

terrane] 

USGS Mesozoic loc. 10168. Admiralty Island, southeastern Alaska; west side of Mole Harbor. Coll.: Edwin

Kirk, August 18, 1918. [Sitka 1:250,000 quadrangle;

Peninsular terrane]

USGS Mesozoic loc. 18927. Kuskokwim Mountains,

southwestern Alaska; 34 miles S. 73° W. of Kashegelok on

crest of spur of ridge east of Cinnabar Creek at a point about

1 mile N. 60° W. of the mouth of the creek. Coll.: W.M.

Cady, 1943, field No. 43ACa39. [Taylor Mountain

1:250,000 quadrangle; Togiak terrane]

USGS Mesozoic loc. 23170. Kuskokwim region,

southwestern Alaska; lat 59°44'02" N., long 160°27'02" W.;

151

/2 miles S. 48° W. of outlet of Kagati Lake; 111/2 miles S.

70° E. of mouth of Klak Creek; 153

/4 miles N., 61/2 miles E.

of head of Good News Lake. Fossils occur near the southeast end of a ridge that lies north of a lake about 2 miles long
19

that drains into Klak Creek; the ridge starts about the center

of the lake and extends northwestward; fossils in float about

900 feet above lake level and 20 feet below the crest of the

ridge. Coll.: J.M. Hoare, August 18, 1950, field No. 50 AHR

590. Recollected by W.L. Coonrad and J.M. Hoare, 1975,

field No. GC4–1122; Goodnews C–4 quadrangle; NE3

/4 sec.

23, T. 5 S., R. 66 W. [Goodnews terrane]

USGS Mesozoic loc. 24504. Colville region, northern

Alaska; west side of Killik River, 12 miles north of junction

with Easter Creek. Coll.: M.C. Lachenbruch, 1949, field No.

49ALac118F. [Killik River 1:250,000 quadrangle; Endicott

Mountains or DeLong Mountains subterrane of the Arctic

Alaska terrane]

USGS Mesozoic loc. D10708. Noyakuk Lake, southwestern Alaska; lat 59°53.1' N., long 158°55.5' W.; limestone within thick section of graywacke and shale; small

outcrop on south shore of lake about 0.5 km east of obvious

promontory; Dillingham D–8 quadrangle, sec. 28, T. 3 S., R.

56 W. Coll.: J.M. Hoare, 1978, field No. 78AHr146.

[Togiak(?) terrane]

USGS Mesozoic loc. D11071, D11075, D11078, and

D11083. Killik River region, northern Alaska; type section

of Otuk Formation on bluff on east bank of Otuk Creek:

D11071, 114 feet above base; D11075, 110 feet above base;

D11078, 107 feet above base; D11083, 101 feet above base

of section. Coll: C.G.Mull, 1975, field No. 75AMu57. Killik River 1:250,000 quadrangle (sec. 31, T. 10 S., R. 16 W.).

[Endicott Mountains subterrane of the Arctic Alaska terrane]

USGS Mesozoic loc. D11284. Kuskokwim Mountains, west-central Alaska; Medfra C–3 1:63,360 quadrangle; float from narrow strip of rubble on south side of ridge;

center E1

/2 SE1/4 sec. 12, T. 22 S., R. 24 E. Coll.: N.J. Silberling, and others, 1980, field No. 80S431. [Nixon Fork

terrane]

USGS Mesozoic loc. D11285. Same locality as

D11284 but bedrock on north side of ridge and 4–5 m stratigraphically higher. Coll.: N.J. Silberling, 1980, field

No. 80-S-432.

USGS Mesozoic loc. D11286. Kuskokwim Mountains,

west-central Alaska; Medfra C–3 1:63,360 quadrangle; bedrock on ridge crest a few tens of meters stratigraphically

above float of middle Norian Halobia and several meters

below float of Monotis (P.) subcircularis and Heterastridium; SE1

/4 NW1/4 sec. 2, T. 22 S., R. 24 E. Coll.: N.J.

Silberling, 1980, field No. 80S436. [Nixon Fork terrane]

USGS Mesozoic loc. D11287. Talkeetna region, southern Alaska; Monotis and Halobia from single 10 cm concretionary bed; Inoceramus? and belemnites from adjoining 1–2

m of section; few tens of meters above top of pillow basalt.

Coll.: N.J. Silberling, 1980, field No. 80–S–382. Talkeetna

C–6 quadrangle (center E edge, NE1

/4 sec. 14, T. 30 N., R. 19

W.). [McKinley terrane]

USGS Mesozoic loc. D11289. Alaska Peninsula,

southwestern Alaska; Karluk C–4 and C–5 quadrangle; limestone 2 miles N. 81°E. of VABM Kek; lat 57°44'30" N., long

155°17'20" W. Coll.: R.L. Detterman, 1980, field No.

80ADt170. [Peninsular terrane]

USGS Mesozoic loc. D12569. Talkeetna Mountains,

southern Alaska; Healy A–5 quadrangle; float from interbedded tuffaceous sedimentary rocks and basalt; center S1

/2

NW1/4 sec. 24, T. 20 S., R. 9 W. Coll.: N.J. Silberling, 1977,

field No.77S152. [Susitna terrane]

USGS Mesozoic loc. D12582, D12583, and D12584.

Northern Alaska; Philip Smith Mountains 1:250,000 quadrangle; northwest side of Atigun River in “Atigun Gorge”;

center W1

/2 E1/2 NE1/4 sec. 23, T. 11 S., R. 12 E.; D12582, at

70 feet in measured section and 1.5 m stratigraphically below

0.5 m calcareous glauconitic sandstone that may be a tongue

of the Karen Creek Sandstone at the top of the Otuk Formation; D12583, at 63 feet in measured section; D12584, at

58–59 feet in measured section. Coll.: N.J. Silberling, 1976,

field No. 76S221. [Endicott Mountains subterrane of the

Arctic Alaska terrane]

USGS Mesozoic loc. D12594 and D12595. Fire Creek

region, northern Alaska; reference section for the Shublik

Formation on Fire Creek, 6 miles N. 80° W. from junction of

Fire Creek with Sadlerochit River, lat 69°31'45" N., long

145°12'20" W.: D12594, basal 1.5 m of Karen Creek Sandstone, 460 feet above base of Shublik Formation; D12595,

unit 6, #2, 450 feet above base of Shublik Formation. Coll.:

R.L. Detterman, 1969, field Nos. 69ADt–106 and

69ADt–105. [Mt. Michelson 1:250,000 quadrangle; North

Slope subterrane of the Arctic Alaska terrane]

USGS Mesozoic loc. D12609 and D12610. Northern

Alaska, National Petroleum Reserve in Alaska; core samples

from West Dease Test Well No. 1; lat 71°09' N., long

155°37' W.; D12609, 3,915 feet depth; D12610, 3,919.1 feet

depth. [North Slope subterrane of the Arctic Alaska terrane]

USGS Mesozoic loc. D12613 through D12617. Northern Alaska, National Petroleum Reserve in Alaska; core

samples from Simpson Test Well No. 1; lat 70°57'12" N.,

long 155°21'52" W.; D12613, 6,318.2 feet depth; D12614,

6,312 feet depth; D12615, 6,310 feet depth; D12616, 6,306

feet depth; D12617, 6,304 feet depth. [North Slope subterrane of the Arctic Alaska terrane]

USGS Mesozoic loc. D13388. Port Graham region,

southern Alaska; north shore line of Port Graham, south of

Selenie Lagoon, lat 59°22.07' N., long 151°51.15' W. Coll.:

Béla Csejtey, 1991, field no. 91ACy–7. [Seldovia B–5

quadrangle; Peninsular terrane]

USGS Mesozoic loc. M749. Alaska Peninsula, southwestern Alaska; on coast near point between Cape Kekurnoi

and Puale Bay about 1

/2 mile NE. from cabin near VABM 96

on Karluk 1:250,000 quadrangle; Monotis occurs about 400

feet above base(?) of Triassic section and 250–300 feet

above coralline limestone that rests on Permian(?). Coll.:

M.C. Lachenbruch, 1959, field No. ADB–123F. [Karluk

1:250,000 quadrangle; Peninsular terrane]

USGS Mesozoic loc. M1020. Upper Yukon, eastcentral Alaska; Charley River A–2 1:63,360 quadrangle;

LOCALITY DESCRIPTIONS
20 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

about 7 miles south of mouth of Nation River; west bank of

Trout Creek opposite mouth of tributary from east; lat 65°05.3'

N., long 141°41.7' W. Coll.: E.E. Brabb, 1960, field No.

60ABa732. [In rocks not included among the Phanerozoic

accretionary terranes]

USGS Mesozoic loc. M1690. Wrangell Mountains,

southern Alaska; nose of spur between head of East Ford

Creek and “Anticline Valley”; 280 feet above base of

McCarthy Formation, measured by Abney level; McCarthy

C–5 quadrangle, central E1

/2 sec.16, T. 4 S., R. 15 E. Coll.:

N.J. Silberling, July 11, 1962, field No. 62-49. [Wrangellia

terrane]

USGS Mesozoic loc. M1691. Wrangell Mountains,

southern Alaska; nose of spur between head of East Ford

Creek and “Anticline Valley”; 310 feet above base of

McCarthy Formation, measured by Abney level; McCarthy

C-5 quadrangle, central E1

/2 sec.16, T. 4 S., R. 15 E. Coll.:

N.J. Silberling and R.W. Imlay, July 11, 1962, field No. 62-

20. [Wrangellia terrane]

USGS Mesozoic loc. M1692. Wrangell Mountains,

southern Alaska; McCarthy C-5 1:63,360 quadrangle; nose

of spur between head of East Fork Creek and “Anticline Valley”; center E1

/2 sec. 16, T. 4 S., R. 15 E.; 415 feet above base

of the McCarthy Formation. Coll.: N.J. Silberling, 1962,

field No. 62-21. [Wrangellia terrane]

USGS Mesozoic loc. M1693. Same as for locality

M1692, but 475 feet above base of the McCarthy Formation.

Coll.: N.J. Silberling, 1962, field No. 62-22.

USGS Mesozoic loc. M1700. Wrangell Mountains,

southern Alaska; McCarthy C-4 1:63,360 quadrangle; collection from single bed about 70 feet below top of cliff-forming unit in lower part of McCarthy Formation on S. side of

Monotis Creek alongside lower rock glacier; lat 61°32.10'

N., long 142°25.75' W. Coll.: N.J. Silberling, 1962, field No.

62-24. [Wrangellia terrane]

USGS Mesozoic loc. M1701. Same as for M1700, but

from single bed about 10 feet stratigraphically lower. Coll.:

N.J. Silberling, 1962, field No. 62-25.

USGS Mesozoic loc. M1714. Upper Yukon region,

east-central Alaska; Charley River A-2 1:63,360 quadrangle;

black papery shale several tens of feet above Trout Creek on

its west side; center NE1

/4 NW1/4SW1/4 sec. 17, T. 3 N., R. 30

E. Coll.: N.J. Silberling, 1962, field No. 62-9. [In rocks not

included among the Phanerozoic accretionary terranes]

USGS Mesozoic loc. M1715. Upper Yukon region,

east-central Alaska; same general locality as M1714, but

10–30 feet stratigraphically higher; lenticular gray-brown

limestone beds more or less equivalent to M1716. Coll.: N.

J. Silberling, field No. 62-8. [In rocks not included among

the Phanerozoic accretionary terranes]

USGS Mesozoic loc. M1716. Upper Yukon region,

east-central Alaska; farthest upstream of pre-Tertiary outcrops; west bank of Trout Creek at water line; dark bituminous papery calcareous shale with Monotis upstream from,

but more or less stratigraphically equivalent to, M1715;

Charley River A-2 quadrangle, SE1

/4NE1/4NW1/4SW1/4 sec.

17, T. 3 N., R. 30 E. Coll.: N. J. Silberling, June 29, 1962,

field No. 62-4. [In rocks not included among the

Phanerozoic accretionary terranes]

USGS Mesozoic loc. M1879. Southwestern Alaska;

Russian Mission C-3 quadrangle; base of bedrock cliff forming north bank of Kuskokwim River about 14 miles S. 89°

W. of Aniak and about 12 miles N. 76° E. of Kalskag; lat

61°34.7' N., long 159°57.4' W. Coll.: W.H. Condon, field

No. 63ACo380. [Terrane asssigment uncertain; possibly

Nyac terrane]

USGS Mesozoic loc. M1898. Keku Strait, southeastern

Alaska; at triangulation station Lee and along strike for

about 1/10 mile on either side; about 130 feet above 63-S152, north shore of Hamilton Bay about 11

/2 miles west of

head of bay. Coll.: N.J. Silberling, June 1963, field No. 63-

S-161. [Petersburg D-6 quadrangle; Alexander terrane]

USGS Mesozoic loc. M1900. Keku Strait, southeastern

Alaska; middle of east side of Hound Island; same locality as

63-S-202, but from thin-bedded limestone about 10 feet stratigraphically lower. Coll.: N.J. Silberling, June 1963, field No.

63-S-202A. [Petersburg quadrangle D-6 quadrangle;

Alexander terrane]

USGS Mesozoic loc. M1912. Admiralty Island, southeastern Alaska; north shore of tidal inlet directly west of triangulation station “Luck” from limestone overlying Triassic

volcanic rocks and 17,750 feet S. 07 E. from triangulation

station “Low.” Coll.: N.J. Silberling and L.J.P. Muffler, July

1, 1963, field No. 63-AMp-259. [Sitka A-1 quadrangle;

Alexander terrane]

USGS Mesozoic loc. M2143. East-central Alaska; collections from traverse taped in N. 45° W. direction along

SW. bank of Yukon River beginning 835 feet N. 50° W. from

rocky point formed of the Tahkandit Limestone; about 6 feet

of calcareous shale beginning at 27 feet traverse distance

above base of section; lat 65°11.1' N., long 141°42.3' W.

Coll.: M. Churkin, 1963, field No. 63ACn 2781D. [Charley

River A-2 quadrangle; in rocks not included among the

Phanerozoic accretionary terranes]

USGS Mesozoic loc. M2156. Northern Alaska; south

bank of Kukpuk River; lat 68°17'25" N., long 165°38'40" W.

22–24 feet stratigraphically below apparent top of vertically

dipping section of “Shublik” [= Otuk] Formation. Coll.: I.L.

Tailleur, 1963, field No. 63ATr 330. [Point Hope 1:250,000

quadrangle; Endicott Mountains subterrane of the Arctic

Alaska terrane]

USGS Mesozoic loc. M2157. Same locality and field

number as USGS Mesozoic loc. M2156, but 27 feet

stratigraphically below apparent top of section.

USGS Mesozoic loc. M2541. Northwestern Alaska;

from measured section at Noyalik Point; 61 feet above base

of section, “Shublik” [= Otuk] Formation; lat 166°11' W.,

long 68°45'30" N. Coll.: I.L. Tailleur, 1964; field No.

64AMm 248. [Point Hope 1:250,000 quadrangle; subterrane
21

transitional between Endicott Mountains and North Slope

subterranes of Arctic Alaska terrane]

USGS Mesozoic loc. M2545. Northern Alaska; Pt.

Hope 1:250,000 quadrangle; Ipewik River; upper 8 feet of

exposed 55 foot-thick section of “Shublik” [= Otuk] Formation; lat 68°24.5' N., long 165°39' W. Submitted by: I.L.

Tailleur, field No. 64AAn61. [Subterrane transitional in

character between North Slope and Endicott Mountains

subterranes of the Arctic Alaska terrane]

USGS Mesozoic loc. M2546. Northwestern Alaska;

measured section of “Shublik” [= Otuk] Formation about 5

miles NNW. of Mt. Hamlet, 0–15 feet below top of section;

lat 166°00' W., 68°48'30" N. Coll.: I. L. Tailleur, 1964; field

No. 64AAn 114. [Point Hope 1:250,000 quadrangle; subterrane transitional in character between North Slope and

Endicott Mountains subterranes of the Arctic Alaska terrane]

USGS Mesozoic loc. M2552. Northern Alaska; Point

Hope 1,250,000 quadrangle; single slab from “Shublik” [=

Otuk] Formation about 8 miles south-southeast of Mt. Hamlet; 165°47' W., 68°40' N. Coll.: I.L. Tailleur, field No.

64ATr120. [Subterrane transitional in character between

North Slope and Endicott Mountains subterranes of the

Arctic Alaska terrane]

USGS Mesozoic locs. M2650, M2651, and M2652.

Northeastern Alaska; Mt. Michelson 1:250,000 quadrangle;

stratigraphic section No. 12 along Fire Creek at front of

Shublik Mts.; SW1

/4SW1/4 sec. 11 to NE1/4NE1/4 sec. 15, T. 2

N., R. 28 E.; M2650, field No. AG 1044, 345 feet above base

of Shublik Formation; M2651, field No. AG 1045, 352 feet

above base of Shublik Formation; M2652, field No. AG

1046, 360 feet above base of Shublik Formation. Coll.: BP

Exploration Co. (Alaska), Inc., 1964. [North Slope

subterrane of the Arctic Alaska terrane]

USGS Mesozoic locs. M5058 and M5059. Northeastern Alaska; section in gorge of Fire Creek, east end of Shublik Mts., lat 69°31.6'–32.0' N., long 145°11.8–12.5' W.;

M5058, 380–395 feet above base of Shublik Formation;

M5059, 275–370 feet above base of Shublik Formation.

Coll.: H.A. Tourtelot and I.L. Tailleur, 1968, field Nos. 68

ATo 2–27 and 68ATo 2–25. [Mt. Michelson 1:250,000

quadrangle; North Slope subterrane of the Arctic Alaska

terrane]

USGS Mesozoic loc. M5113. Northern Alaska; old

cutbank on east side of Ipnavik River, 10–12 feet above base

of the limestone member of the Otuk Formation; lat

68°40.6' N., long 157°3.5' W. Coll.: H.A. Tourtelot, 1968,

field No. 68ATo 23W. [Howard Pass 1:250,000 quadrangle;

Endicott Mountains subterrane of the Arctic Alaska terrane]

USGS Mesozoic loc. M5115. Northern Alaska; same

locality as M5113; upper 3 feet of the limestone member of

the Otuk Formation. Coll.: H. A. Tourtelot, 1968, field No.

68ATo23Z.

USGS Mesozoic loc. M6624. Northeastern Brooks

Range, northern Alaska; Fire Creek, 6 miles N. 80° W. of

junction with Sadlerochit River, lat 69°31'45" N., long

145°12'20" W; unit 6. Coll.: R. L. Detterman, 1970, field

No. 70ADt-105. [Mt. Michelson 1:250,000 quadrangle;

North Slope subterrane of the Arctic Alaska terrane]

USGS Mesozoic loc. M6657. Alaska Range, southern

Alaska; 2.25 miles southwest of the Eldridge Glacier at elevation 5,110 feet; lat 62°58'25" N., long 150°07'50" W.; Talkeetna D-1 quadrangle; section 25, T. 22 S., R. 14 W. Coll.:

Bruce Reed, 1976. [Chulitna terrane]

USGS Mesozoic loc. M6671. Southwestern Alaska; lat

59°37'22" N., long 160°38'55" W. Coll.: W. L. Coonrad and

J. M. Hoare, 1976, field No. GC5-1739. [Goodnews C-5

quadrangle; Goodnews terrane]

USGS Mesozoic loc. M6674. Northern Alaska; east

tributary to Saviukviayak River, lat 68°57' N., long 148°08'

W. Coll.: H.A. Tourtelot, 1968, field No. 68ATo18. [Philip

Smith Mountains 1:250,000 quadrangle; North Slope

subterrane of the Arctic Alaska terrane]

Stanford Univ. loc SU 3652. Alaska Peninsula, southwestern Alaska; Karluk C-4 and C-5 1:63,360 quadrangle;

east side of Puale Bay about 2,300 feet N. 40° W. of VABM

Hike. Coll.: W.T. Rothwell, Jr., 1962, field No. ROC 1113.

[Peninsular terrane]

Manuscript approved for publication August 29, 1995

Published in the Central Region, Denver, Colorado

Photocomposition by Patricia L. Wilber

Graphics by Springfield & Springfield

LOCALITY DESCRIPTIONS
PLATES 1–11

[Contact photographs of the plates in this report are available, at cost, from U.S. Geological Survey Library, 

Denver Federal Center, Denver, CO 80225]
24 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA

PLATE 1

[All figures natural size]

Figures 1–14. Monotis from USGS Mesozoic loc. M1912 (collected from single 0.5 m bed 

along with more than 45 species of other bivalves, ammonites, gastropods, and 

other invertebrates of the Cordilleranus Zone).

1–3, 11–12. M. (Entomonotis) cf. M. (En.) ochotica densistriata. All left 

valves.

4–10, 13–14. M. (Pacimonotis) subcircularis.

4–5, 9–10. Right valves.

6–8, 13–14. Left valves.

15–16. Halobia and Monotis from USGS Mesozoic loc. M1900 (collected from a few 

meters of bedrock).

15. Halobia cf. H. fallax.

16. Monotis (Eomonotis) ?pinensis. Left valve.

17–21. Monotis (Pacimonotis) subcircularis. USGS Mesozoic loc. M1898.

17, 20. Right valves.

18–19, 21. Left and right valves.

PLATE 2

[All figures natural size]

 Figures 1–7. Monotis (Monotis) alaskana.

1–6. SU loc. 3652.

1–2. Left valves.

3–6. Right valves.

7. USGS Mesozoic loc. D11289 (approximately same stratigraphic 

level as SU loc. 3652). Left (top) and right (bottom) valves.

8–13. Monotis (Pacimonotis) subcircularis. USGS Mesozoic loc. M749 (stratigraphically lower than SU loc. 3652).

8–10. Left valves.

11–13. Right valves.

PLATE 3

[All figures natural size]

 Figures 1–3. Monotis (Pacimonotis) subcircularis.

USGS Mesozoic loc. M1691.

1. Left valve.

2, 3. Right valves.

4–7. Monotis (Monotis) alaskana. USGS Mesozoic

loc. M1692 (32 m stratigraphically above loc. M1691).

4, 7. Right valves.

5, 6. Left valves.

8–13. Monotis (Monotis) ?haueri. USGS Mesozoic loc. M1693 (18 m stratigraphically 

above loc. M1692).

8, 11–13. Right valves.

9–10. Left valves.

14–15. Monotis (Monotis) ?alaskana. USGS Mesozoic loc. M1700. Left and right valve.

16–20. Monotis from USGS Mesozoic loc. M1701 (collected from single bed 3.3 m stratigraphically lower than loc. M1700).

16–17. M. (Eomonotis) jakutica. Left valves.

18–19. M. (P.) ?subcircularis. Right valves.

20. M. (P.) subcircularis. Left valve.

PLATE 4

[All figures natural size]

 Figures 1–5. Monotis (Pacimonotis) subcircularis.

USGS Mesozoic loc. M1690 (10 m stratigraphically

lower than loc. M1691). Wrangellia terrane.

1–3. Left valves.

4–5. Right valves.

6–8. Monotis from USGS Mesozoic loc. D12569 (collected from float).

Susitna terrane.

6–7. M. (M.) alaskana. Right and left valves.

8. M. (P.) ?subcircularis. Left valve.

9–10. Monotis (P.) ?subcircularis. USGS Mesozoic loc. M6657. Chulitna terrane.

11–13. Monotis (Monotis) salinaria. Siriuskogel, Salzkammergut, Austria, the type

locality of the species. Figured for comparison with Alaskan specimens of M. 

(Monotis).

11. Left valve.

12. Right valve.

13. Right and left valves.

PLATE 5

[All figures natural size]

 Figures 1–4. Monotis (Pacimonotis) subcircularis.

USGS Mesozoic loc. D11285 (bedrock collection). Nixon Fork terrane.

1–3. Left valves.

4. Right valve.

5–10. Monotis from USGS Mesozoic loc. D11284 (float from about 2-m-wide strip of 

rubble exposed through tundra vegetation and 4–5 m stratigraphically below 

D11285). Nixon Fork terrane.

5. M. (Eomonotis) ?typica. Right valve.

6. M. (Monotis) sp. indet. Left valve.

7–10. M. (Eomonotis) jakutica.

7. Left valves.

8–10. Right valves.

11–16. Monotis (Eomonotis) anjuensis. USGS Mesozoic loc. D11286 (bedrock collection 

several meters stratigraphically below float of M. (M.) subcircularis and Heterastridium sp.). Nixon Fork terrane.

11–14, 16. Right valves.

15. Left valve.

17–22. Halobia and Monotis from USGS Mesozoic loc. D11287 (single bedrock slab, possibly redeposited into Jurassic or Cretaceous flysch). McKinley terrane.

17. Halobia cf. H. fallax.

 18, 20–22. Monotis (Eomonotis) ?pinensis. Width to height ratios tectonically 

deformed.

18, 21. Right valves.

20, 22. Left valves.

19. Monotis (Eomonotis) ?pinensis and Halobia cf. H. fallax.

PLATE 6

[All figures natural size]

 Figures 1–5. Monotis (Eomonotis) daonellaeformis.

1–4. On slabs with Halobia fallax (for example, upper right of fig. 4), USGS 

Mesozoic loc. M1714.

5. Left valve from USGS Mesozoic loc. M1020 (approximately same 

locality as loc. M1714).

6–16. Monotis (Eomonotis) ?pinensis and the associated ammonite Himavatites.

6–11. USGS Mesozoic loc. M1715 (several meters 

stratigraphically above loc. M1714).

6. Right and left valves.

7–8. Left valves.

9–10. Right valves.

11. ?Himavatites cf. H. multiauritis.

12–16. USGS Mesozoic loc. M1716 (approximately same stratigraphic level as 

loc. M1715).

12, 15. Right valve.

13, 14. Left and right valves (left valve on right side of fig. 13).

 16. Slab with the associated ammonite Neohimavatites cf. N. 

canadensis.

PLATE 7

[All figures natural size]

Figures 1–3. Monotis (Pacimonotis) ?subcircularis. Non-accretionary Monotis-bearing rocks, 

Upper Yukon region.

1–2. Interfering shells and impressions from USGS Mesozoic loc. M2143.

3. Left valve from USGS Mesozoic loc. 9382.

 4–5. Monotis (P.) subcircularis. USGS Mesozoic loc. 24504. Endicott Mountains

subterrane of the Arctic Alaska terrane. Two views of single bedding surface.

PLATE 8

[All figures natural size]

Figures 1–8. Monotis from the type section of the Otuk Formation.

1–2. Monotis (Pacimonotis) ?subcircularis. 

Interfering shell impressions. USGS Mesozoic loc. D11071.

3. M. (Eomonotis) ?typica. Right valve.

USGS Mesozoic loc. D11075 (1.2 m stratigraphically below

loc. D11071).

4. M. (Eo.) ?typica. USGS Mesozoic loc. D11078 

(0.9 m stratigraphically below D11075).

5–8. M. (Eo.) typica. USGS Mesozoic 

loc. D11083 (1.75 m stratigraphically below loc. 11078).

5–7. Right valves.

8. Left valves.

9–17. Monotis and Halobia from measured section of Otuk Formation on the

Atigun River.

 9. Monotis (Monotis) haueri. Right valves.

USGS Mesozoic loc. D12582 (1.5 m stratigraphically below 0.5 m of calcareous glauconitic sandstone that may be a tongue of the Karen Creek Sandstone at the top of the Otuk Formation).

10. M. (Eomonotis) ?typica. USGS Mesozoic 

loc. D12583 (ca. 2.0 m stratigraphically below loc. D12582).

11–17. Halobia and Monotis from USGS Mesozoic 

loc. D12584 (ca. 3.5 m stratigraphically below loc. D12582).

11–12. Halobia cf. H. fallax.

13, 15, 17. Monotis (Eomonotis) typica. Mostly right valves.

14, 16. M. (Eo.) typica. Left valves.

PLATE 9

[All figures natural size]

 Figures 1–4, 9–11. Monotis (Entomonotis) pachypleura.

1–4. USGS Mesozoic loc. M2552 (collected from single slab of float). 

Left valves and one right valve (fig. 2).

9–11. USGS Mesozoic loc. M2545 (uppermost 2.5 m of Otuk Formation, 

as exposed). Mainly right valves; left valve in center of fig. 9

 5–6. Monotis (Pacimonotis) subcircularis. USGS Mesozoic loc. M5115. Right 

and left valves.

7. Monotis (Eomonotis) ?typica. USGS Mesozoic loc. M5113 (less than 3 m 

stratigraphically below loc. M5115).

8, 12–13. M. (Entomonotis) ochotica ochotica.

8. USGS Mesozoic loc. D12581 (Otuk Formation, Tiglukpuk Creek). 

Left (lower) and right (upper) valves.

 12–13. USGS Mesozoic loc. D10999.

PLATE 10

[All figures natural size]

 Figures 1–6. Monotis from USGS Mesozoic loc. M5059 (a 17 m stratigraphic interval separated 

by about 6 m from top of the Shublik Formation at Fire Creek, the reference section 

of the formation; see also pl. 11, figs. 1–10).

1–4, 6. M. (Eomonotis) ?jakutica. Left valves and right valve.

5. M. (Eo.) n.sp. aff. M. (Eo.) inaequivalvis. Left valve.

7–12. Monotis (Eomonotis) obtusicostata and associated M. (Eo.) ?jakutica. USGS Mesozoic loc. M6674 (collected from single bed).

7–9, 12. M. (Eo.) obtusicostata. Left valves, especially figure 8 and upper right 

side of figure 12; right valves, especially upper center of figure 7 and 

lower figure 12.

10–11. M. (Eo.) ?jakutica. Left valves on the right sides of both figures, associated with M. (Eo.) obtusicostata.

13–14. Monotis (Eomonotis) obtusicostata. USGS Mesozoic loc. M6624 (from within 9 m 

stratigraphic interval separated by about 6 m from top of the Shublik Formation at 

Fire Creek; within few meters of same stratigraphic level as loc. M5059, figs. 1–6). 

Left and right valves.

PLATE 11

[All figures natural size]

 Figures 1–12. Monotis from Fire Creek, the reference section of the Shublik Formation (see also 

pl. 10, figs. 1–6, 13–14).

1–3. M. (Entomonotis) ochotica ochotica. USGS Mesozoic loc. D12594 

(basal 1.5 m of Karen Creek Sandstone). Left valves and right valve (fig. 

3).

4–5. M. (En.) ?ochotica ochotica. USGS Mesozoic loc. D12594 (same as 

figs. 1–3). Left valves.

6–7. M. (En.) ?ochotica ochotica. USGS Mesozoic loc. M5058 (essentially 

same locality as D12594). Left and right valve.

8–9. M. (Eomonotis) ?pinensis. USGS Mesozoic loc. D12595 (about 2 m 

stratigraphically below locs. D12594 and M5058). Left valves.

10–12. M. (Eo.) obtusicostata. USGS Mesozoic loc. M6624 (about 7–8 m 

below loc. D12595). Left valve (fig. 10) and right valves (figs. 11–12).

13–15. Monotis from core samples, West Dease No. 1 test well, National Petroleum 

Reserve in Alaska.

13. M. Monotis (Pacimonotis) subcircularis. USGS Mesozoic loc. D12609 

(depth 3,915 feet). Left valve.

14–15. M. (Entomonotis) ochotica ochotica. USGS Mesozoic loc. D12610 

(depth 3,919.1 feet). Right and left valves.

16–20. Monotis from core samples, Simpson No. 1 test well, National Petroleum Reserve 

in Alaska.

16. M. (Eomonotis) pinensis. USGS Mesozoic loc. D12613 (depth 6,318.2 

feet). Left valve.

17–18. M. (Eo.) n.sp. aff. M. (Eo.) anjuensis. USGS Mesozoic loc. D12614 

(depth 6,312 feet, fig. 17) and USGS Mesozoic loc. D12615 (depth 6,310 

feet, fig. 18).

19–20. M. (Eo.) typica. USGS Mesozoic loc. D12616 (depth 6,306 feet, fig. 19) 

and USGS Mesozoic loc. D12617 (depth 6,304 feet, fig. 20).